The two littlest ones probably wouldn't get much out of Six Flags and it does cost a chunk to get in; if you did go there, though, there's also a waterpark and you'd probably want to do that in the heat of Summer. Amusement parks make a large percentage of their profit off concessions once you are inside the gate, and for that reason don't usually have water fountains, you need to be prepared to spend hotel prices for soft drinks or water once you get inside the gate, or be very well hydrated going in the door. The Zoo, Arch, museums etc. would probably fill your time for the trip and are all more closely grouped, Six Flags is well outside the I-270 beltway, several miles west down I-44.
