Do you have a realtor? I can recommend a couple of people with offices in Hopkinton and neighboring towns. I live in the next town of Westborough and can attest to the attractiveness of the area. You will find Southborough quite expensive and probably with fewer rental options than Hopkinton, Westborough, Northborough, Shrewsbury and Ashland. All of these are close to Interstate 495 and the Mass Pike (Interstate 90). There are also some very nice apartment/townhouse complexes in the area which is another option, at least in the short term. I realize "moving twice" is not ideal but you will not have problems getting a place. Any realtor will have great on-line photos of every home for sale or rent, and then you can come out maybe once and see 6 or 8 places plus get some tours of the towns with a knowledgeable expert. They will want to help you rent because they hope you will come back to them as a broker when it's time to buy! (I know they work for the seller but still, they are looking to build a client base.)
Most of the towns of early childhood options for recreation programs, library story hour, and so on. It's a good way to start to make connections. Many towns also have a Newcomers' Club with activities for adults and kids - play groups, adult social nights, etc. Many towns sponsor a babysitter training course and it helps you find sitters in an area you are just getting to know. Sometimes the high schools have a job listing area in the guidance office and you can get sitters or mothers' helpers that way.
Is your 5 year old in school yet? If so, you will meet lots of people through parent groups and class activities. It's not impossible to find preschool options even at mid-year - people move away just as people like you move into the area, so slots open up.
There are some great museums in the area without going to Boston, so there are things to do if the weather isn't conducive to outdoor nature areas. Even the existing areas (Hopkinton and Ashland State Parks, plus numerous areas managed by local land groups, Audubon, etc.) are easily accessible even if you are one or two towns away.
Winters get tough here in January and February, more than December, so you should be able to move without a big risk of snow. You'll need warm outerwear, of course, and a few snow shovels or a plowing service, and you'll need to factor in heating costs vs. AC costs in California! I think you'll be able to get the kids excited about the new surroundings - definitely show them pictures of the New England Aquarium and the Museum of Science (Boston), the Children's Discovery Museum (Acton), the Ecotarium (Worcester), the Roger Williams Park Zoo (Providence), and Cape Cod beaches. All are accessible from Central Massachusetts. They'll meet a few kids and will be acclimated in no time!
Meantime, let them know they will keep their favorite bed, blanket, stuffed animals, toys, etc. If you're comfortable doing so, promise them something for the new house - could be a toy (maybe a sled?), could be a new thing for their rooms (wallpaper border, bookshelf, whatever) that will get them excited. Downplay losing their friends - tell them about Skype so they can talk to their friends all the time. Play up exploration of new things. If they ask if they can come back and visit CA, say "yes, at some point" or "Yes, when we can...." or "that would be fun, wouldn't it" and don't say "No it's too expensive". They will lose interest after a few weeks in their new exciting place and after meeting new friends.
Feel free to contact me if you're coming into the area to visit, and I'll give you my phone number!
