Pretty sure all the stations have elevators, and are required to do so by ADA. Usually some are broken, but escalators will get you up.
I think most of the kids were age 5+ before we started taking them into the city, and even as 20-somethings, they burn out on sightseeing in a few hours. I took the day off work to take my 17, 19 year old nieces to a few museums plus lunch in Chinatown. I thought we had a fabulous time but they didn't want to do it again in their week long visit with grandparents. When I tried dragging them to museums in their city, I heard that their usual routine is an hour in the museum, two hours in the gift shop, lunch in cafe and then home. There's no way I'm paying a museum admission fee, $10+, and then leaving in an hour.
Some of the historic sights might be more toddler friendly, I'm thinking mount Vernon and Harper's ferry, both of which are kind of like colonial Williamsburg with replicas of life in the old days and butter churning and metal forging demonstrations. Baltimore aquarium is good at that age, and used to have a cheap Friday evening rate.