If you are looking to just bang around the lake with your kids and their friends you want a Hobie 18. The capacity will be fine. The price on a used boat will be nice. Best of all the thing is bulletproof. This is key if you are going to let the kids take it out by themselves eventually. The N20 and the Hobie 20 are not exactly A cats but they are more fragile than what you want. You are not going to miss 2 feet.
A Dart 18 may also be a good option but I don't know much about those boats.
Don't even think about a Hobie 21. It's way too big for what you want to use it for.