We live outside Boston,and my daughter and her hubby did this cruise in August and loved it.
If this was me, I would, get the balcony for yourself and husband,and an inside for the kids.
Given the age of your kids, they will hardly be in the cabin except to sleep. Your youngest may spend more time with you, but, I bet he won't. I don't think the inside should bother the kids at all,and, the money you save, can be used for fun, family things.