Yes, there is lots to do. I have three sons, who were, in our fist family trip there in 2010, the same ages as yours. They have varying interest in aviation but they all loved KidVenture. It is very hands on, very immersive, and they get two hours credit towards their A&P certificate! Go upstairs in the museum and you will find more fun things for the kids. There are also various kid friendly activities throughout the event. The one thing you can't do is leave them in Camp Scholler…boring. My middle son, who is not aviation minded, is sitting right next to me saying that he loved seeing the blimp, the airshow ("awesome"), "the museum was pretty cool." (literally…on a hot Wisconsin summer day.) As great as Wittman field activities are, you have to have a means to escape. Swimming has been mentioned. There is not only the waterpark but, amazing as this may sound, Lake Winnebago actually has a beach. Get them off the airfield for dinner, at least once in awhile, even if you plan to grill at your campsite. Oshkosh is the home of outlet malls, so let your daughters indulge for a few hours (that is, if you can't convince them that the hands on experience of actually building an airplane is the be all and end all.)
We will be camped at Warbirds all week. Come by and see us and I am sure that my boys will be glad to tell them what is fun.