A couple of oddments come to mind.
Firstly, you can assure your daughters that nobody knows how gravity works, some of the cleverest people in the world are still trying to work it out. All anybody really knows is what gravity does does, which is pull things together - the bigger and closer they are, the more it pulls.
Secondly, whilst Fountains of Paradise is really for grown-ups, Arthur C Clarke wrote two excellent science fiction books for children. The first is called Dolphin Island which is about people living in the sea in the future, whilst Islands in the Sky is all about living on a space station. He explains the science quite well in both, and I *think* that a 12 year old would enjoy both, although the first is slightly easier reading (and with pictures!).
G