FB - how old is your kid? Is he responsible enough to bring along a pier on the canal? Next to Sandwich Marina is OK to soak some worms for scup/choggies/flounder - go near slack...
--- Edit - just saw he is 2 1/2. I would recommend highly the hatchery (below) but as far as fishing, I would find a local Sunny pond, bobber and some worms...
If you are there - You MUST take him to the Hatchery in Sandwich:
http://www.mass.gov/dfwele/dfw/dfwdistr.htm#Hatcheries
Bring 5 bucks in quarters and have your kid throw then in and see the fish explode
