I am a very utilitarian traveler and would refuse to travel with a heavy burden.
I have always been a city Explorer, rather than a beach goer. When my daughter was little, I would refuse to leave my hotel each day with anything other than the bare minimum. I figured that I was not to go about into an Amazon expedition, and that if I *really* needed something I could stop and buy it. I also noticed that I seldom used all of that stuff I was carrying back and forth.
I do not recall what other destinations you will be going to, but for the one I know, I recommend against the big backpack. First, the city is hard on travelers. Blocks are long, the humidity hits you hard. There is a street vendor in every corner and you can find inexpensive snacks too. I am not sure if I would want my kids lugging weight around. Do they need to snack thru the day?
My solution: A decent size "manpurse" for your husband, so he can put his wallet and necesities and perhaps a couple things for the kiddos and a decent size crossbody bag for you. The kids can knot their light jackets around their waists when it gets hot.
Another disadvantage of a backpack is that those give easy access to pickpocketers.