I think with the right parental guidance/supervision this could be a very fun toy.

From the website it seems like a great way to generate some black and white outlines that kids can still color in. If used like that it seems almost strictly more creative than a coloring book, no? There are plenty of other ways kids can express creativity with pre-made art too. Maybe they use them to illustrate a story they dreamed up? Maybe they decorate something they built with them?

Also, some children might want to have fun be creative in ways that don't involve visual arts. I was never particularly interested in coloring or drawing and still believe myself to be a pretty creative individual. I don't think my parents buying me some stickers robbed me of any critical experience.