6-8 year olds are active little creatures who aren't going to want to sit still. Writing is still new to them and most of them won't have the skills to write good sentences. They don't really pick up writing as an art until after eight unless they are very talented - I've met a few of these kids but there really aren't many.
Here are some ideas for a few activities:
1) Grab a great big piece of paper - HUGE - and get markers and pencils and ink brushes and pencil crayons and have them all work together to make a giant map of their own creation. Don't stifle anything - even if they are copying a TV show or comic or whatever. As they do this, have them discuss what they are doing in a free flow type of way. What their land is like, who lives there, how their land works with or against their neighbors land, etc. This will probably take one or two sessions to finish depending on the kids.
2) Put out costume pieces for when the kids walk in. Let them explore the costumes and hats (hats are great). Tell them to think about a character who would live in the land they created and dress like them. You could even do face paint if you were so inclined. Have them introduce themselves in a talk-show type game. One kid can interview and the other answers the questions - everyone else is the audience.
3) Have the kids draw their characters and fill in a character sheet (name, favorite colour, what the character likes doesn't like etc.) Also make sure they find out their characters Goal, Motive, and Conflict. Goal - what they want, Motive - why they want it, Conflict - what is stopping them. Have them present their picture and talk about their character. They will be able to elaborate more verbally at this age than in writing.
4) Talk about story structure using a picture, talk about how conflicts happen and build to a climax, then resolve. See if you can make a verbal story together using story structure and all the characters in the world you have made.
6) Have them act out the story they wrote as a group. Film it. Write it out. The kids say the best lines when improving.
7) Explore senses using things in paper bags for touch, things in bottles for smell, cool things to see for sight, music instruments for sound, and safe food for taste (keep in mind allergies). Have them create a group senses dictionary to add to their writing.
8) Have them make an outline of a story for their character by walking the whole group through the steps. Then have them turn that outline into a book (an actual mini book all stapled together) and add touch to it by having them make pictures for the book that they can collage different feeling material.
9-12 year olds are full of ideas. They are pushing the boundaries and trying to shock. They want to know how far they can take things. Even if they say something shocking - don't shut them down unless it is bullying (telling someone their idea is stupid), morally harmful (drug use), or physically harmful (falling off the chairs repeatedly). Instead either ignore the negative behavior (the more attention you give it the worse it gets) or if it's an attempt at humour or legitimately trying to figure something out, ask the kid to elaborate and dig deeper with them. Make them really think about why a character would do something like that.
Kids at this age still need to move around. They are capable of writing and sitting still, but not for great lengths of time. They should be up on their feet at least every twenty minutes for an active activity that has to do with writing.
1) A lot of the same ideas that were outlined in the 6-8 group work well with all the ages - so go ahead and use them. I've seen 19 year olds have a blast over world building and dressing up and interviewing characters. Those things never get old.
2) You can be a little more formal with this age group. I always like to start with character and goal, motive, conflict. Having them draw out their character and fill in a character sheet is a great way to start the creative process. I also like to have them do the same for their antagonist.
3) A great setting game is to have the kids draw a setting with no characters in it, then muddle them up in the middle and pass the pictures out randomly. The kid then has to put their character they made in that new setting and write a scene for them. Remind them that each character always has a goal, motive, and conflict.
4) Show plot structure and teach them how to write an outline. Have the kids write a point by point outline for a story.
5) Explore genre by listing all the genres they can think of then creating a wheel of genre (a spinner where all the genres are listed (and decorated). This can be used later for lots of things. Alternately all the genres can be put into a hat for randomization. You can then have the kids tell a circle story using one or more sentences each where, as the story progresses, they each pull a genre out of the hat (or spin the spinner) and carry on the story in the new genre. This way a story starting in romance might change to a spy story, then a drama, and a sci-fi, and then switch to fantasy. It's really funny.
6) Make a group story by creating some kind of reason all the characters would come together. Find out how they all might relate to one another. Ask them what their characters reaction to the situation would be. Have them each write a scene based on this.
The 13-18 year olds are a more serious crew - or at least more relaxed and maybe sleepy. They are usually happy to write for half an hour at a go - some maybe more. Have some instrumental superhero theme music or the like playing during writing time to keep them focused. This age group likes collaboration as an idea factory then prefers to go off and create their own thing out of it. They are a lot more critical of their work and more reluctant to share. Their egos are on the line. You have to be gentle with this crew. They can also take adult type instruction - not dumbed down. But remember, this is summer camp and school is over - so have fun.
1) You can do everything with this group as suggested in all the other age groups. They love it.
2) Try to add a random element to the planning stage. Such as, they randomly draw the name of a character out of a hat (not even getting to choose boy or girl) and then have to build a character out of it using a character sheet. Or they randomly get a genre to write about. Or they randomly get a plot. In RIO kids' writing club we play an Anime creation game where (In pairs) the kids randomly draw a typical anime character, a typical anime location, and a typical anime goal. Then they have to come up with a title, story line, and theme song. It's super funny.
3) A way I've used senses with the older kids is to have them write a setting paragraph with some action (a scene from their story). Then I do the senses activity with them and have them create their own sense dictionary. After that I get them to rewrite their paragraph adding in senses. It's really changes things for the better. Senses bring the reader right into the scene.
4) We also work on hooks, middles, and endings in this age group. First they need to learn story structure, and outlining. After that I have them write a hook for the beginning of their story - something startling or unusual that would make the reader ask what is going on. I then have them write the climax of their story (with out having written anything else of it). Then they write the final sentence. After that, all they have to do is fill in the missing bits and they have a story.
5) You could play the game "Why would they say that?" Have the kids write random lines on pieces of paper and put them in a hat. Have the kids write a dialogue scene either on their own or in pairs. At a certain point have them draw a phrase from the hat and write it as character dialogue. Now they have to some how justify that line in the scene.
These are just some of the ways you can bring play into writing. Kids learn through play (even the big ones) and they learn more completely when moving. Play is a no fail thing. You can't fail at play - so it takes the pressure off. Just think of the things you want to do with the kids and then find a way to make at least part of it active. If you want to teach a lesson - try to find a way for the kids to learn the lesson on their own without you telling or lecturing them. Discovery is the best teacher and then they start explaining it to each other. Not everything is possible to do like this, but the more you put the control in their hands the more actual learning gets done.