I had the same question regarding pools and young children and consulted both a lawyer and my insurance agent. Their responses echo what has been posted here...
-Most states do NOT allow a parent to waive the right of a minor to sue. So in general a waiver has zero value.
-Follow a hierarchy of pool saftey - showing a good faith effort to pool safety will help you if you're ever sued, and its just common sense to have a safe environment around a pool
1) Always have adult supervision - if you have to go to the bathroom or answer the phone, the kids have to leave the pool area too
2) Fence and lock your pool
3) Swimming lessons for the kids, especially the ones that live with the pool