Just go to the beach and walk along the boardwalk like everyone else.
The Salisbury Zoo is a nice 1-2 hour stopover on the way in, but for
the age of your kids I'd say the beach, bars, and amusements there are
probably more appropriate. Supposedly there are things like GoCarts
in the area just outside of the main island which they would probably
like. I don't know what direction you will be coming from, but I also
like to stop in Cambridge at the visitors center and walk around as
well. Worth stopping there just for the bathroom.
http://www.tourdorchester.org/attrac...attraction=348