I found a nifty little site, it’s children, teacher, and family friendly, called Storybird
It starts with art. You pick a painting by one of the site’s artists and write a story about the picture. It’s a great exercise if you find yourself stuck. Since the site is aimed at children and educators a lot of the artwork has a fairy tale and children’s book vibe.
There is plenty of art to chose from and you can also read stories by other authors of all ages. There’s even the option to print your books.