I live in the Boston area.
It is a really great place to live and rear children - people are friendly, communities are lovely. I've been here more than 20 years, have never been the victim of a crime, have never seen a gun other than in a policeman's holster, and have made great friends.
But housing is very expensive and getting worse. And the commute into Boston/Cambridge can be hellish from the further-out suburbs.
When we moved we borrowed a friend's house for a week or so and then rented. Most tenancies are for a year and most are unfurnished.
There are some really excellent school districts in the Boston area - if you decide on the move, pm me and I will give you a start on some towns that might work - depends on where you are working.
Most kids do summer camps or have a summer nanny - we've done both.
You should ask the company's HR department for help - if they really want you they will help. They are probably also used to helping people relocate.
I can't advise on the visa situation but there is a whole bunch of stuff your dh will need to do in order to be able to practice in the US and it is very different to the NHS. There are also other options for him - working in biotech for example (huge industry in this area) running clinical trials etc.
The most important thing in your package (apart from the money) is imo health insurance. Massachusetts has mandatory healthcare (thank you republican governor Romney) that is currently obamacare but isn't dependent on it so healthcare position is less uncertain here than other places but it is vital you have good insurance. If you do, the healthcare is second to none. Your children will see a paediatrician for yearly well-child visits and be referred to specialists easily if needed, you can have second or third referrals easily. My dogwalkers' partner has been diagnosed with a lung disease - she was seen by a specialist and then referred on to someone who sub-specialises in this disease and is also taking part in a new clinical trial. Dental insurance is usually offered too and means you can have twice-yearly check ups, and a bit of braces covered.
Also if you are in a teaching position, make sure it is tenure track.