How to get them writing: Let your kids tell you all the ways you are wrong
Write a few sentences about a "Perfect Day" in the life of your child. Only, instead of writing all of your child's favorite things, choose all of the things they dislike in your description.
Present your writing to your child with a "teacher's pen" to correct all of the places where you got it wrong. They will have fun crossing out your mistakes and replacing these words with descriptions of their favorite things to do on their "Perfect Day".
Tip: include lots of details about their LEAST favorite foods, hobbies, and activities. They will easily find these "errors" and will be quick to correct you!