If your daughters like theme parks, there is one in Beijing located on subway line 1, called Baijiao amusement park. However the park is located out in nowhere, but it is damn cheap.
http://www.urbanrail.net/as/beij/beijing-map.htm
Ah well there are much better things to do in Beijing than heading for an amusement park, but then I like to head out to areas not mentioned in any guidebook... it also has a fake Disney castle.