You're definitely going to have to have a winter kit and a summer kit given your northern climate. I agree with Rikkrack that good footware has to be a foundation for all the bipeds that will be traveling under their own power and it has to be appropriate footware for the season. I'd toss in a box of toe warmers and hand warmers, enough so everyone could have two sets. 20 miles in one day is absolutely not realistic for anyone that does not train for the distance. So that presumes spending the night out.
My winter kit includes a mid-weight poly base layer, fleece pants, a fleece jacket, wind-proof pants, heavy wool socks, winter gloves, and an N2B flight jacket (parka). Multiplying that by five is going to take up a lot of space, but the alternative is not moving at all and having to hunker and wait for help to arrive.
I have the advantage of having a garage, so I can keep about 5-7 liters of water in my car that only freezes a few days a year. Without that, water can be a real challenge. For food, I keep it extremely simple: Granola-type bars (they don't melt), Millenium Bars (5 year shelf life), and sports bars and gels. I do have a small solid fuel stove and metal water bottle along with chicken bullion cubes, but I've been rethinking this and will probably go with an alcohol stove and/or tea lite candles in the future because the exhaust given off by esbit stoves is noxious. Realistically, the stove and hot soup is just for morale. Everything else if food cube food and just meant to keep me alive. I have historically carried several MREs, but their summer shelf life is awful in a car (really, one summer), I don't like the taste, and would rather be able to eat on the move. Still, some items like ranger bars, lemon poppyseed poundcake, and the crackers are worth considering, but the entrees seem like a waste of space and time.
One kid-transit idea is a shopping cart, however, more and more supermarkets are going to auto-locking wheels so they're worthless when you leave the parking lot.