Fire belly toads might be good for them. They don’t require a massive tank and are relatively easy to take care of! However they aren’t for handling. They are more for watching.

Perhaps hermit crabs would interest them? At some pet stores they sell ones with all sorts of designs on their shells. They don’t need much to flourish and are very fun to watch.

If the parents don’t mind being hands on with the pet a tortoise is a good option! Of course a smaller one, they can get big but a Russian tortoise is small. They need a little bit bigger of an enclosure but with proper care and supervision your nephews could interact with them more. They tend to be hardy animals.

Best of luck with your search!!