I've had the baklava from Zaxaroplasteon in Arlington, and thought it was too dense and too sweet (and way too big a portion), but all of those objections might appeal to a kid. ;)
I do like the baklava at Greek Corner, and the one at Helmand (which, they will correct you, is "bucklawa"). I think all those little grocery stores in Watertown carry it too -- are they Armenian? A different kind of traditional.