Vee, there are tons of things to do in and around Boston. There are duck boat tours (these vehicles go on both land and water), swan boat rides at Boston Common, the New England Aquarium (which is on the smallish side, perfect for smallish children), there are also lots of museums, some geared specifically for kids like the Boston Children's Museum, which is known for its hands-on, interactive exhibits. Faneuil Hall Marketplace is a fun place to shop and eat, though it might be a bit challenging with a 3 year-old.

If they want to venture outside Boston, the Cape and Martha's Vineyard are beautiful (though very touristy and crowded).

I've linked to the duck tours below, here are a few others:

Boston Children's Museum: http://www.bostonchildrensmuseum.org/exhibits-programs

Faneuil Hall Marketplace: http://www.faneuilhallmarketplace.com/

I'll look around a bit more and ask my kids what they liked best in Boston. Hope this helps!

Here is a link that might be useful: Duck Boat Tours

I lived in Boston many years ago. I, too, recommend the Boston Common, and walking around that area. Boston is a great place to walk, in general, downtown. Be sure not to miss Beacon Hill, with its quaint alleys and charming, old architecture. There are sometimes concerts along the Charles River. I used to enjoy riding the subway over to Cambridge, as well.

Not to restart the Revolution, Vee, but it is a short trip to Bunker Hill, site of "the shot heard 'round the world." It's an outdoor venue, so the little one could run around. There is also the Old North Church, of "one if by land, two if by sea" fame, and Paul Revere's house, and the Old State House that has what we were told has the only lion and unicorn in the U.S. on the front of the building. The U.S.S. Constitution, our oldest commissioned ship and most famous for her actions during the War of 1812 against Great Britain, is free for touring. My daughter and I were the only ones who were short enough to stand upright below deck. Your children will possibly hear a different version of history than they have been taught at home!

We ate lunch at Legal Seafoods and enjoyed it. There are lots of seafood restaurants in the city. Hope they enjoy the visit.

Thank you Sheri, Mary and Carolyn for your recommendations. The children's museum looks very hands-on and river trips are always a favourite especially in hot weather.
Japanese d-in-law really appreciates good seafood places and son has been forced to get used to the very 'healthy' diet. He was never fat but is now seriously skinny!
Carolyn, interesting about the Revolutionary War.
Over here we take quite a pragmatic view of those events. I think it is because in our long history we have had so many wars. Won some lost others; tick them off the list, maybe learn something from them . . .'though probably not enough.
I had two great great great ancestors in that conflict, one on each side. They seemed to get on fine afterwards!
Will there be any Teabagging going on in the harbour or is that a political no-no too far? ;-)

Please, you must know we are very into the ecology scene. Mustn't litter or pollute with that tea tossing stuff.

Vee - When I visited Boston, I spent a good part of one morning exploring their public library. It's gorgeous. Might be difficult with a 3-year-old however.

Also, I loved the Legal Seafood restaurants. My travel companion and I ate at 3 of them during our week in the city. They were all different, architecturally, but had equally good food.

If the weather is nice, there are the Boston Harbor Islands. You can go out by boat and visit the lighthouse and go to some of the other islands. There are also whale watches and a motor boat harbor thrill ride called Codzilla.

The Museum of Science is great for kids and has a butterfly exhibit. The Duck Boats leave from there.

Here is a link that might be useful: Boston Harbor Islands

One other thing, there is a children's book that's very famous in Boston called Make Way for Ducklings by Robert McCloskey. There is a statue dedicated to it in the Boston Public Garden, where you can also enjoy the Swan Boats. When my children were in elementary school, they had a field trip where they followed the path the ducklings took, and then went on a Swan Boat ride.

Here is a link that might be useful: Make Way for Ducklings

merry, thank you for the extra information. Somewhere we do have an ancient copy of Make Way for Ducklings sent to my children by a US Aunt. I'll try and find it (easier said than done) and mail it to little Emma. She is too young to read yet but will enjoy the drawings.
Son and family have rented an apt. on Union St as they felt it would be easier than a hotel-stay and no more expensive.
The harbor islands and boat trips look very inviting especially as the weather is likely to be quite hot.