How in world do you get your child to write? This is battle cry of many parents. A lot of imagination, with a little bribery (or praise) is all you need to get your child writing. We'll supply imagination. The praise and bribery is all up to you.
Grocery List: Enlist your child's help in making grocery list. Walk around kitchen, naming things you need from store. Ask your child to write everything down. Your child can also suggest foods you might need from store and he can add those, too.
Old Checks: If you've recently switched banks and have checks that need to be destroyed, first let your child play with them. Give him some envelopes and he can pretend to pay bills -- while getting him to do some writing. Of course, destroy checks afterwards. If you do not have checks available, you can just give your child some blank pieces of paper and he can make his own checks.
Cards: If your child is interested in Pokemon or Yu-Gi-Oh cards, then give him some index cards cut in half and have him design his own cards. Encourage your child to give characters names and special abilities on their cards.
Fictional Journal: Sometimes it is hard for children to write in a journal. What is there to write about? Instead of a standard journal, give your child a fictional journal. He or she can pretend to be anything they'd like to be and write a journal as that person. Your child could write from perspective of an Astronaut discovering a new planet, Prince or Princess on an Adventure, Archaeologist finding a new species of Dinosaur, Famous Athlete, President of a Country, Passenger on Titanic. Your child could write from perspective something instead of someone, a mailbox, an animal, a pen. The possibilities are endless.