When we were just there we originally got 3 FP+ for things like Big Thunder and Space Mountain but we have a 2 year old and needed to do a child swap and found out for child swap they only wanted/needed a FP scanned for 1 adult and then gave you the return ticket for up to 3 riders. Things like Space Mountain / Test Track take at lease 30 minutes for a group to come out so both adults using a FP+ in the 1 hour window can be tough.
What worked for us for Space Mountain was to book 2 FP+ for adult/older child and the other 1 FP+ (or 2 FP+ if your too small rider is over 3 years) for an attraction near it like Tommorrow Speedway or Buzz's ride at the same time window as Space Mountain. Then use the child swap ticket for the other adult to take older child again.