St John's Wood question: too sleepy for 'urban' family moving to UK?

Thanks in advance for any advice.

Family with 3 kids coming to London from US for 3-5 years.  We currently live in Back Bay part of Boston, so we're used to row houses with a tiny outdoor space and relying on the nearby city parks for the outdoors.  We're lucky recipients of a generous expat package, so housing cost isn't a big deal -- we were perfectly fine with the places we recently saw within our budget.

Long story short, if it were just up to me and my wife, we'd look for a place in Notting Hill or Holland Park or the like b/c we like the feel of those neighborhoods, my commute to Euston station vicinity would be easy, and the few friends we have in London live nearby.  However, looks like at least one of our 3 kids will be in school up in Hampstead and the others in Notting Hill/Kensington since our move came together very recently.  I was told that traffic up to Hampstead schools in the AM will be very bad.

That has us thinking about St John's wood as a place kinda in between.  Seemed like a lovely neighborhood but maybe not exactly the kind of urban living we really like.  

Thoughts on St John's Wood?  Other suggestions?  Thx!

I work in St Johns Wood.  It's not gritty in any sense, but it's right next to Regents Park, has a nice High Street (what we call a main street for shopping) and is very international.  As someone who also lived in Boston (Watertown), I wouldn't say it's like Back Bay, but you won't find that here really anyway - it's not Boston.  It's not sleepy per se, and it's all of 5 minutes into central London by the Jubilee line. 

The question is - are you going to have a driver, going to drive to the school, take a bus or the tube?  That will really determine where you should live.  Have you thought about Belsize Park?  It's on the Northern line so easy for Hampstead & Euston, and the other kids (if taking the tube) will be able to get to their areas reasonably easily via tube.  Belsize has some great shopping/eating, near the Heath and has a lot of row houses.  Also easy to get into central London.