Thanks, Lilliana, for the great play-by-play
and also to Melda for the advice. There is an all-day Montessori near us, and that's definitely something to consider for next year since that program isn't available for 3yos. I wouldn't have thought of that, so I appreciate your mentioning it. But we prefer the school where he's planning to attend in the fall because it's more established as a Montessori and has a sunnier, more peaceful school environment. I did ask already our school about the all-day program, and they explained the 5yo's have the classrooms in the pm for other activities when the little ones are gone.
Re our decision about what to do in the afternoons, I should say that "daycare" doesn't do the place justice. It really is a fantastic center where ds has thrived--it's in a university setting and is more of an early childhood learning program than just daycare. Their philosophy has some elements of Montessori, but also incorporates Reggio Emilia, M. Garber, problem-solving approaches from "High Scope," etc. He's already been going there and it's familiar to him, so we're most likely going to keep a few pm spots there. We can always pull him out if it doesn't work, but if possible we'd like him to keep him in a place where he already has a lot of friends since he will be with new kids in the am.
Anyway, I do appreciate the responses, and some of our friends think we're pushing it, too. Other friends managed to do a similar arrangement and think it's worth trying. I can post again later in the year to let people know how it turned out if cmlp or others are considering this in the future. Right now, we're just undecided about how to handle the 90 min gap between the two programs...