The cover charge for Jellyrolls as of when I was there in August is $7 (and worth every penny). It is 21 plus, and they are very strict about it--I've watched them turn families away.
And it is a lot of fun. I never miss it on my trips (I'll be going all five nights of my trip in December). It won't be cheap, but perhaps you can look into leaving your little ones at one of the child care rooms in the area for a couple of hours (I believe the Boardwalk has one, and I know the Dolphin and Beach Club have them). If you do look into doing that, note that the entertainment doesn't start until 8:00.
Karen