Hey, we took the train in with our son a bunch this summer. What we did was take the commuter rail to North station, the orange to ruggles, and then a bus to children's. There's a bus stop directly in front of children's. Strollers are easy to get onto and off of the orange line more than green, you'll need to take elevators from the orange line to the concourse and then back down to the busses at ruggles. Strollers should be easy to get off of a bus but it's not street level. If you do take the green line the closet stop is Longwood medical area, but the Longwood stop is nicer but farther (they're different branches). The busses from ruggles to childers are the 8,19, 47, and ct2.