LP Steamers! We live around the corner and frequent it. One word needed....CASUAL. You can order crabs by the half dozen. Plenty of parking on the street and @ 5:30, lots of parking.
On the other side of the harbor in Canton is Captian James' landing. They offer a deck on the water. They have parking but are a bit more expensive.
Just in case you don't know, Mama's on the Halfshell is a seafood place and not a Crabhouse. They will have crab dishes but no crabs. I don't think it is upscale, I always wear jeans, but it is by no means a uber-casual place. The dining room upstrairs is decorated more upscale, but the atmosphere is still casual. Downstairs is casual. I think it depends on your kids ages and what time you go, you would probably be fine for an early dinner. I have never seen people bring kids there, but I have only been for brunch or a late dinner.
What ages are your kids?