Depends how much "entertainment" you and your nephews require apart from snorkelling but another option might be Ko Ngai. There are some upmarket resorts there now and the scenery and beach are stunning but still no shops, roads, ATMs etc. AFAIK. It has the advantage of being within easy reach of two other islands, Ko Kradan and ko Muk. The former has only basic accommodation but the beach is even more stunning and the latter has the Emerald Cave which would make an exciting day trip for kids.

You can also do an overnight camping trip on Ko Rok if you're feeling adventurous (the resorts on Ko Ngai organize them, you don't have to bring camping equipment yourself). Ko Rok is one of the few places you're still pretty likely to see sea turtles in Thailand nowadays.

You don't say when you're going though....the Andaman side is better for the winter months, Ko Tao during the (European) summer.

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“Not all those who wander are lost.” Tolkien.
Life is not measured by the number of breaths we take, but by the moments that take our breath away.