Bruce Van Patter is an illustrator, author and 'motivator' of young writers. He has a website full of ideas of how to get children motivated to write. The button will take you to the section of his website dedicated to families full of ideas to try at home.
Taken from the Bruce Van Patter site, this is a particularly fun tool to get your children writing stories. They simply press on the generator button and they are provided with a wacky headline for which they have to invent a news story.
Storybird is a website that allows your child access to professionally drawn artwork that your child can use as picture prompts to write a story. You then have the option of getting it printed and delivered to you as a book. This can be very motivating for children to feel like real authors.
There is a free option that allows you to write and publish (you do get charged for prints and delivery) and as well as a paid subscription that provides you with more options.