Which Disney theme park?

im going to Florida soon and my family and I are going to Disney for one day. which park should we go to? there are two adults, a sixteen year old and a ten year old. we were thinking animal kingdom but don't know of there is enough to do for a whole day. we have been to magic kingdom many times but the kids are getting old and we want to experience more in Disney? we don't know about epcot either. please help us decide and explain why.