Guppys and Mollys. It'd be impossable for them to be killed off. Get 2 female and 3 male of each. Maybe a small catfish for cleaning dutys that you know will be neglected eventually. Mollys are atom bomb proof, Guppy males (as already said) are pretty and the two will live together very happily.
The best thing about the Molly/Guppy combo' is the 99% chance of them breeding. A perfect way for kids to learn the facts of life and to see the most exciting (in my opinion) part of the hobby!
If the tank over-populates tell the dad to put in a lone Bala Shark to eat up any fry.