I don't think people tend to uses baby swap for anything that you can bring a baby on such as small world for instance but they let you do it for any ride in which someone would have to stay behind with the baby and ride afterward. Any ride which has Fast pass generally will of course too.
We used child swap on Splash Mountain. The CM didn't have any of the passes (?) so she gave us Fast Passes that we could redeem at any time. MIL and DH went first while DD played in the Splash play area (GREAT place!) Then DH and I went right in using the Fast Pass while MIL watched DD. Worked great!