We live in Buffalo too and love Ocean City, MD. It's a little bit of a further drive but we like the town, and will sometimes visit Dover, DE on the way there or the way back.
Friends of ours with small kids, also from Buffalo, will visit Cape May, NJ and they enjoy it. I've only been to Asbury Park, and it wasn't for the beach, but didn't seem to be a ton going on there. Similarly, I've been to the casinos in Atlantic City and outside of the immediate area, it seemed rather dangerous and sketchy. I don't have a lot to base it on, just what I recall.
Another area I'd consider is Cape Cod. About the same driving distance and you can add on a few days sightseeing in Boston, or at least use Boston as a back-up on a cool or rainy day if you have one. A girl I work with goes to Cape Cod with her family every year and they rent a small house. They love it.
Have fun planning!