Cartoonize.net is a useful website that takes a photo you upload or choose from the web, and cartoonizes it. Simple. You can download the result and save it to your computer. You can also link from the main site to other editors which put an image on a magazine, or with the Simpsons etc.
There are other ways to cartoonize. Your own photo software may have it as a feature under effects. Befunky is an online image editor that offers it as a feature. Picnik, another online image editor, doesn't have cartoonize as an effect but has many similar ones. However, I found Cartoonize does the job quickly and easily with no sign up required. The top pic is from Cartoonize, the lower one from Befunky.
I think this could add some fun to images your child or students want to use in a story. You could cartoonize some holiday snaps and weave them together into a short narrative. Try cartoonizing pictures of favourite toys and inventing captions for them, or take a journey into the world of comic creation and add dialogue in speech bubbles with inexpensive software like Comic Life.
Great family project for the school holidays!