So in the scenario you note, you would need to use Rider Swap in order for DS to ride again. However, from my personal experience, you would not need to use your FP+. You could if you were trying to burn it I’m sure.
Your 2.5 yo will not be able to pre-schedule FP+(and will not ever need any) but there is a “hack” that you could schedule same day FP+ for them to get extras for the family.
There are a lot of different scenarios you could use here to string something together but it can require a lot of mental effort! I would recommend just booking the FP+ for things that people actually do want to ride and then think of Rider Swap as a bonus.