When I finally found the toddler play area, it took a couple trips for her to acclimate, but now we just have someone take her directly there and things work out really well. T. and the other adult go do whatever T. feels like doing and/or can be convinced to try, until he starts lobbying to go home, at which point we meet up in the toddler area and hang out there for a little while longer.
Today we got going fairly late, so it was a little after noon when we left the museum. R. suggested Cambridge Brewing Company for lunch, since it wasn't too hot for me to tolerate the patio. Service was a little slow, but they brought out T.'s food first, which helped. The kids were wonderfully well behaved the whole outing. It was exactly the kind of Saturday spend time as a family that I have been wanting us to have, and I'm finally feeling like we're getting there.
Later, R. put A. down for a belated and very long nap. He slept for an hour, then I woke him up so he could get a bike ride with T. [ETA: Since I'm correcting this long after the event, I don't know whether it was my son T. who took the nap, or my daughter A. That is some serious word-swapping. The bike ride would have been with my husband R., obviously.]
It probably didn't hurt that T. and R. slept in until almost 10 a.m. this morning.