Those ages seem perfect for Atlantis. They would absolutely love it.
Are you perhaps a Costco member?
Several years ago, I put together a package for my birthday to go there. My brother-in-law does a lot of work there and tried to get me a "deal".
I went to Costco Travel and their package was way less expensive than rack rate or what he was able to get me. Additionally, if the budget is tight, you can stay right next door at I believe it is a Comfort Suites and purchase day passes to Atlantis.
Atlantis on a BudgetIf the budget is loose, there are some really great high end packages as well.
IF you go, splurge for dinner at GrayCliff Inn and take the wine cellar tour. And eat with the locals in the Fish Fry area. Food at Atlantis is overpriced and under good, by and large.
Graycliff Inn