Collaborate on a screenplay. Involve friends and family members to film it. Have your child write a letter of complaint to that company. Sometimes the company will send you free stuff. In the same vein, have your child write a thank you letter after having received wonderful service.
Many people find it easier to compose on the computer.
This works well for brainstorming and editing. Encourage your child to use the computer for all written assignments. If you don't have a computer, you can often times use decent computers for free at school or the library. Go over your child's writing assignments before they hand them in. Use this writing guide to evaluate written work.