I highly recommend that all of you watch this video on packing. And you all need to read the whole Travel Tips section on Packing Light.
Although the typical 22" roller or backpack carry-on works for most international flights, it is unlikely to be a legit carry-on for flights within Europe. When you check out your airlines, you'll need to pay attention to weight restrictions for carry-on, too.
Whether it's a backpack or a roller bag, I limit the weight to 20 pounds or less TOTAL. That means including the weight of the bag. Some bags, especially wheeled ones, are much heavier than backpacks. I'm hoping for much lighter next time with my new Eagle Creek Afar Backpack. Last year I traveled with the Eagle Creek Load Warrior Wheeled Duffel Carry-On. If you look at the specs for the weight, you can see how much lighter the former is than the latter.
So, you all need to try on some bags somewhere, filled with enough to weigh about 20 pounds, and walk around the store, go up and down stairs and try to imagine carrying that weight for significant distances. Put them on, take them off, lift them over your heads to put them somewhere like on a train rack or in a plane overhead bin. No one will be doing that for you and don't you volunteer to handle the luggage for the rest of your family members! Everyone should be able to manage their own luggage.
Confession is good for the soul: I kept the weight down, but I managed to put so much stuff in that EC rolling duffel that I could barely get it in the sizing bin at the airport. I'd swear that British Airways sizer is smaller than Delta's!
About the packing cubes: I love them. My husband hates them. I have both regular and compression style which I used last year. I like those, but they only take the air out, not the weight, so there can be a tendency to pack more than you really need with them. Although most people roll their clothes in them, I've found that my sizes are too big for that to be efficient. I fold and have plenty of space in them for my tops and pants.
I hope that you find something that works for you all. I have an RS convertible backpack, but it doesn't "fit" me very well. The new one linked above works great on me. You may find that one kind/size doesn't fit all 4 of you.