You could use FP+ to allow all 3 kids to re-ride with dad. Get FP+ for everyone in the family. Mom & 3 kids use their FP+ for the first ride while Dad waits with the 4th child. Then Dad still has his FP+ to use. Dad uses his FP+ and 3 kids use the RS Pass to ride again.
We did this last year for almost all the rides we planned to RS since we had a group of 5 riders and 1 toddler. It just means that whoever waits with the youngest child can't take them on another ride using FP+ but we always found something for them to do that didn't require a FP.