Well, for anything the five year old is too short for, you'll be able to get a rider swap pass. The swap at boarding is for when there's a child who's tall enough to ride but doesn't want to and can't wait alone. One adult would wait with that kid at the boarding area while the other adult goes on with any kids who do want to ride, then they swap off. They will *sometimes* allow this as well if there are more adults than kids under 7, since under 7's can't ride alone, but it doesn't always work.
If the situation is 1 adult, a 10 year old, and a 5 year old, they are unlikely to allow the adult to ride with each of the kids since the 10 year old can ride alone. You can always ask, but they may not allow it. Another option is to just wait in line separately, once with each child, or schedule FP for all of you including FIL. Take one child with your FPs, then come out, switch bands with FIL and take the second child.