I side with you on staying north this time of year plus I've always wanted to see around the Great Lakes area of the country, Upper Peninsula of Michigan. I'd use a trip like your's as an excuse to do that if time allowed. Good luck and have a safe trip which ever path you follow.
I know what your girls mean about wanting a "warm" ocean from growing up in Oregon. What the Pacific Ocean lacks in heat in the Northwest is makes up for in beauty though. It still would have been nice to go for a swim in it as a kid without nearly freezing to death in the middle of summer. Now that I'm older I don't have as big of an urge to want to put on a swimsuit as when I was a kid. Hopefully your girls get to spend plenty of time playing in the "warm" ocean.