This was a good reply I received to a similar question:
If you want to ride the same ride again, request a
DAS return time when you first enter with your FP+. Ride with the FP+, and depending on wait time, you may be able to get off and back into line with the DAS.
If you want to ride something different than the FP+ ride, go to that different attraction and request a DAS return time. Then go ride the FP+ ride. Depending on wait times, you may be able to immediately go back to the other ride and use the DAS return time.
If the child with a DAS is too short for some rides, and/or won't ride certain rides, you and he can get a DAS return time for something else while the rest of the party rides with FP+ or waits standby.
Regardless of DAS use, once your party uses all 3 prescheduled FP+, you can book another FP+ one at a time.
The DAS can only have 1 active return time, so after you ride with the DAS, you can get another return time (same ride or different). You can do whatever you prefer while waiting -- maybe use a FP+, eat, shop, potty-break, people-watch, etc.
I cut and pasted it to my notes so can't give credit to the kind person who responded.