Re: [MA] New England Aquarium to Boston Children's Museum.
I've never done that walk specifically, but I don't believe it would be a problem to walk it. Maybe 15-20 minutes? I don't have kids so I'm not sure how long they can walk without getting cranky and tired though -- Join the DSLR Kiva team!
Yes, those roads should all have sidewalks and stuff. You can look on Google Street View if you want to get an idea of the sort of roads they are. They're pretty busy, but have sidewalks. -- Join the DSLR Kiva team!