I live in Sherborn, which is next to Natick. I would think if he has the flexibility to leave for work at 10am he'll be able to get to Children's well before 11am easily on most days. Otherwise if he was going during rush hour it can be awful traffic. I don't recall which T stop Children's is at but if you go to
MBTA.com > Official Website for Greater Boston's Public Transportation System there are trip planner tools you can use to find the fastest route. I can tell you a few things about taking the train from Natick:
1) The downtown Natick station only allows parking for Natick residents so you need a sticker. You can check out the MBTA site to see how many parking spaces there are. If I have to go into Boston I usually just have my husband drop me since we aren't Natick residents. (or I drive to Wellesley)
2) The W Natick stop doesn't have much parking and tends to fill up early.
I think if you took the train to Sudbury it would really only be practical if you're near one of the Concord stops.
Anyhow, that's my two cents! :-)