Many thanks to everyone for the very helpful and detailed replied. FTers are the best! ^

I had forgotten about the possibility of parking at the MoS, (obviously too long since I was in Boston) and think that might be their best plan. It's something they would all enjoy, and if it's a good day to be outside, they can pick up a Duck Tour from there also. It's a pretty quick trip (roadworks permitting) from there to Logan for the flight. And if the kids are tired from a busy day, hey, they're more likely to sleep!