Welcome, Paul!
FastPass+ selections, especially when combined with the need for
Rider Switch, can be a little confusing, but as a mom of three who has used it often, I can certainly help you with your planning!
For your particular scenario I would suggest choosing
FastPass+ selections for an attraction with Rider Switch for two of the adults in your party. This way you can obtain a Rider Switch Pass for the remaining adult who will be able to wait with the children. When the first two adults are finished riding the attraction, the third adult will then be able to use the Rider Switch Pass in order to ride. This way all adults are able to enjoy the attraction that the little ones aren't quite big enough for and one adult is left with an extra
FastPass+ selection they can use on an attraction along with the 4-year-old's
FastPass+ choice.
I hope I have been able to clear the process up just a bit for you, Paul, but please let me know if you have any further questions or concerns.
Happy Planning,
Lauren