We have rented a cottage on the Maine Coast for a week each summer during the past few years. In each instance, the rental was direct from an owner (no "rental company" involvement whatsoever). It's considerably less expensive without the middleman involvement of an agency. If you want (if so, please advise), I can ask the "director of reservation details" here (...I just write the checks) for the Internet contacts for some of those owners and I could then PM the info on to you. Posting such info in open forum here would likely be inappropriate and might appear to be advertising (it would not be intended to be -- I'm not on commission and I don't even know these owners...). Anyway, ask and ye shall receive some info via PM.
I don't know your family situation, children (if any) ages, or your personal prefererences (and Maine is a very big state with a whole lot of coastline). However, in very general terms I'll just point out to you that coastal Maine south of Portland is far more densley populated and more "urbanized". On the other hand, once you get "downeast" of, let's say, Hancock, you are headed into the "boonies" (although beautiful, it's nonetheless the boonies). I'd be focusing on the extensive coastal area in between, personally. Just my opinion and two cents' worth, having spent at least a little time over the years in much of the ME coast all the way to the Canadian border.
By the way, coastal ME is where fog was invented and later perfected.
I can highly recommend The Kennebunks, The Yorks, Wells, Ogunquit and a small town called Perkins Cove, and all are very kid friendly. They are all within about a 30-40 minute drive of each other, unless, of course, you choose the scenic route along Coastal Route 1.
I harbor no prejudices whatsoever on the matter and truly couldn't care less, but at least be aware that Ogunquit is well known for a significant "alternative lifestyle" presence --- in the same way as Provincetown, MA is so known. Whether or not you regard that fact as being compatible and consistent with the above quoted claim of Ogunquit being "very kid friendly" is, I guess, a matter of individual, personal viewpoint.