This has come up before in chat, so I figured I'd address it here for anyone interested.
Using FP+ and rider swap passes have some interesting caveats that can be exploited. Only the first person riding needs to scan and use a fast pass to get a rider swap pass. If parent A and B both had to the fast pass line with parent B holding the child, parent A scans their magic band and rides like normal while parent B gets a rider swap pass and waits. Once parent A returns, the child is handed off and parent B can use the swap pass ###OR### both parents can approach the fast pass line with parent A now holding the child. Parent B scans the FastPass while parent A collects a rider swap pass. Each parent got to ride once with the FastPass reservation and also holds a swap pass to use at a later time or date.
This can be taken a step further by each parent making a different fast pass reservation at the same time. For example Splash Mountain and Big Thunder mountain. Make both reservations for 1000-1100. Parent A rides Splash using a fpp and B collects the rider swap. After the ride, parent B passes of the child and rides big thunder using a fpp. Baring some major ride shutdown causing the window to expire, both parents should be able to ride using their own fpp, and then can ride the opposite ride at their convenience using the rider swap pass.
This can be done in epcot with test track and soarin to get around the tiering, but requires more hustle in between rides.
Clear as mud??!