Bar Harbor is interesting if your toddlers are not squeamish about mud.
The tide goes out, leaving a huge muddy tide pool area with small shells and some creatures (tiny crabs).
My toddler hated this and would only sit on a boulder while I shelled around him. He almost had a sandal float away. My 4 year old who was shelling with his dad had a lot of fun.
My cousin who recommended that we visit Bar Harbor was inspired to get a PhD in part because of a few summer vacations at Bar Harbor.
There are other DCUM posts on Maine and kids. Regular Google will surface them.