If you have not yet made your reservations for the White Mountain area and Acadia it may be a moot point. You are also picking one of the worst possible weeks from a traffic standpoint to travel. From North Conway, NH, to Bar Harbor is just over 200 miles. How long it will take will depend on your route (there are multiple) and traffic but it should be less than your 5-6 hour timeframe. July 3rd being a Friday will compound this. It could be a miserable trip trying to get through it all. If you can travel to Acadia mid-week that would help some. There's plenty to do in the area to keep you occupied for the extra time. Your trip does not allow for a lot of "down" time to enjoy what the area has to offer and will be compounded by the extra traffic due to the holiday. With kids I'd drive straight through VT and spend the time in NH & ME.