This is what we often do: We are a family of 5, kids aged 7, 6, and 2. Determine which rides are child-swap eligible. Know which rides the big kids will and won't do (DD7 refuses to ride ToT, for example). So we get only 2 fastpasses for ToT, use the other 2 park tix for another FP at another child-swap ride in the park, say RnRC. So, we go to ToT, say we need a child swap pass, me and DD and DS2 sit behind the coffee cart, DH and DS6 go on ToT with FP. When done, I take DS6 and turn in the child swap FP. We do the same with RnRC, or whatever rides at whatever parks. All told, my kids get to ride each ride twice, DH and I do the ride once each and DS2 runs and plays like the wild man he is. We get more FP rides done since we are splitting our park tix on the child swap rides.
As well as teh child swap pass being good for up to 3 or 4 people, on occasion I have received passes with an expiration a few days out or (twice!) with NO EXPIRATION! If we end up not being able to use the pass, I always give them away. Well, except the no exp ones!