It might be fun to get a simple, durable waterproof Point & Shoot camera. I have one from Panasonic that's decent and under $150.
I know some kids who had a blast with the waterproof camera, whether it's just at the pool or out in the rain. Plus, composite lenses and seals make it a lot more abuse resistant than your average phone or tablet, which could be a good thing with any 6 year olds I've known.
Things like this might be an option:
Bright colors help it not to get lost in the laundry too. :-)