It's totally a full day place. It's very spread out and you take a bus from location to location. We got the regular passes and didn't have time to do everything included in our admission price. There are IMAX movies and simulators, lots of exhibits and things to see. They even have times when you can meet an astronaut! I would have LOVED it when I was your girls' ages! Look online at their site at all the things you can do on just a regular pass. We didn't think it would be so big or that there would be so much to see. We could have easily spent several hours more, and come back a second day. I believe most of the passes are good for 2 consecutive days, so if your girls get tired or you need some beach time, you could go back the second day to see the rest. One thing to know: the food is ridiculously expensive. It makes Disney look like a bargain. Be prepared! They also search all your bags and you have to go through a metal detector. The lines were long to buy tickets when we got there, so you might want to purchase them in advance if you're going on a weekend day. A lot of the coupon books for the area have coupons for KSC admission as well.
