The water in Maine will be cold no matter what month you decide on. The water temps on the Atlantic are usually in the high 50's that time of year. Lakes will be a little warmer than the ocean. NH and Vermont will have lake water temps a bit better. Usually in the 60's to high 60's. CT on the sound will have water temps in the 60's. If you are concerned about your kids, most kids don't care if the water is a little cold.
Although the water in Maine is pushing it. If you are going for the beauty, it's hard to beat Acadia. I'm paranoid about the cliff sections of the park with my kids. Several years ago we were witness to someone falling off one of the cliffs. At least the last time we were there, there were no rails on the edges. That's just me though, there are about 2-3 million visitors every year and only about 2 people die per year.