The northern New England states each have a maple syrup weekend. Check to see if it coincides with your visit and look for places where you might try sugar on snow and take a sleigh ride. Young kids can have a lot of fun simply sledding or ice skating although late March might be more mud season than snow. It all depends. Check out the Montshire Science Museum in Norwich VT and the Billings Farm Museum in Woodstock. They're fairly close to each other. Snowshoeing and tubing are popular at some winter resorts and ski areas. Google NH snowshoe trails for some suggestions.

For your time in Boston, try to book a hotel that has a pool for the kids to spend some time at. Boston has great museums for kids, there is a Children's museum and the museum of science is fascinating for kids of all ages as is the aquarium, Harvard's Peabody museum of natural history, all right in the area. If the duck tours are running, that might be fun for a few hours too. As for the rest of the trip, if it isn't too cold or snowy, Cape Cod can be fun, not for swimming in the ocean, but for walking along the beach, maybe a whale watch, visit to the National Seashore, walk through Provincetown, stay at a motel/hotel with a heated indoor pool.

emalloy: I couldn't have said it better!

March is too early for whale watches.

That's right. Whale watches start out of Boston early April and start mid-April out of Ptown.

This is all great -- thanks so much! The conference is at the Boston Harbor Hotel, so we'll be staying there through my husband's work. From the website, I think it's right in the area of the attractions you've mentioned, all of which sound right up our alley. (And in our experience, the pool is KEY -- it's almost all they need. Lol)

I looked into the maple syrup weekends -- great advice! -- and found that they will indeed be happening the week that we're there. In particular, the VT open houses are March 26-28. I found an inn that looks charming & great for kids in Lyndonville, VT -- the Wildflower Inn -- does anybody know it? Not sure what the weather will be like up there that time of year, but if it's anything like Chicago I imagine it's hard to predict. Snow would make things easy -- kids love to sled and skate, and Burke Mountain (right next to the Wildflower Inn) looks like a mom 'n pop ski area where we could put the kids in their first skis. Mud season...well....not so good.

I've lived on a dirt road. If it's well maintained it's not too bad during mud season but it depends on how quickly the road thaws and gets wet. If it happens slowly, then it's not bad at all. But if it's bad, well, you just deal with it. Around our area it happens around March 14/town meeting. Being forewarned, wear the proper boots. Their website looks nice. Although I think there's a college in Lyndonville, it's still a small town. After all, it's the Northeast Kingdom. It's a long drive from Boston so I would look for an activity mid-way to let the kids stretch.

Practically next door to the Boston Harbor Hotel is the New England Aquarium. A few blocks the other direction is the Children's Museum. And just a quick cab ride is the Science Museum. All three will entertain kids at that age and are very interactive. You can also go skating on the Frog Pond on Boston Common. It still may be operating in March.

March in Boston is probably the most unpredictable month for weather.

Good comments from dfrostnh. Here's a few tidbits:
The thaw up in Lyndonville is usually in early April. Boston to Lyndonville is 3 1/2 hours. There are two routes - I-93 to I-91, or I-93 to I-89 to I-91. The second is 20 miles longer but goes by the Montshire Museum of Science in Norwich VT. www.montshire.org That would be a great stop with the kids.
The Wildflower was just named the Best Inn in New England and Burke Mountain was named #3 for Best Ski Mountain by Yankee Magazine readers http://www.yankeemagazine.com/issues/2010-01/interact/10things/readers-choice
Hope you enjoy Boston and your travels in New England.

If you're baseball people (My Sox are Red, yours are maybe White?), you can get a tour of Fenway Park. They run every day and since games don't start until April, you might get lucky and be allowed to walk on the field. My six year-old LOVES skating on the Frog Pond on the Common (www.cityofboston.gov can get you the schedule) and if your kids are into history, Boston By Foot does a one-hour kids' walking tour called "Boston by Little Feet" led by an interpreter in period costume. Also, if you register to www.bostoncentral.org, you can see tons of family suggestions and you'll get an email update with family-friendly events in the city and surrounding communities that week. Enjoy your visit!