We visiting Acadia National Park in the summer. I think late spring, summer and early fall are the best times for families to visit. We flew from Boston into Hancock County- Bar Harbor airport located about 15 minutes outside of Bar Harbor. Alternatively, Bar Harbor is about 1.5 hours from the closest international airport in Bangor, ME.
If you do fly into Bar Habor airport, there is no need to rent a car. There are regularly scheduled shuttle busses that run into Acadia Natinoal Park from Bar Harbor every day and Bar Habor is a great walking city. We spent 5 days in the area but could have easily spent another week with plenty to do and see.