If you're doing the heavy work, I'd suggest a commercial/civilian backpacking bag. Something greater than 3500 cubic inches (uh, about 70 liter). Just watch the weight; the guideline is that a bag shouldn't be more than 1/3 your weight. In my teens, while Scouting, my pack could be close to 1/2 for a week-long trip, but that was when I was young.

Your wife may just have to tough it out and carry some of the kid stuff for your youngest- bottles/bags, formula, diapers, wipes. Essentially your diaper bag. I'd avoid anything with a single sling, like purse, bag, duffel. If you have to walk any appreciable distance you're going to be hurting from that.

Your boy can probably hold a 10-15 lbs. pack. Food, some fun items, a bottle of water.

I'd focus on water purification, food (and procurement), navigation and shelter. since you're in MN I wouldn't worry about packing gallons of water; maybe 3 liters, and your son and wife with 1 each. You can stop and filter as needed. A good 4-person tent should be OK.