I have a great 3 minute writing activity for for kids...I call it braindropping.
Pick a subject such as "winter" and tell your kids to write winter at the top of their paper. Tell them that their pencil is going to become a race car and that you are going to time them for three minutes.
When you say, "go" tell your kids to write every thought or word that falls out of their minds that reminds them of winter. There's only one catch...they can't lift their pencils from their papers. If they reach a point where nothing is coming to mind, tell them to write, "I can't think...I can't think..." Believe me, something else will come to mind and soon they'll be back to terrific ideas.
This activity works well for young kids (perhaps K-1) when you do it orally. Same concept...just move around the group and ask for words. Once again, "I can't think" is just fine.
You'll be surprised to see the free flow of background knowledge tumbling onto your student's papers or out of their mouths.
When the kids are done, let them share their lists (words, thoughts, and phrases are fine).
Another good idea would be to have them braindrop about a book they know. Just have them write the title of the book at the top of the paper. Have fun!
Don't forget, this is the activity that I do every single time I start a writing project. You can do it too!