(Probably others have already been doing this, but new for me.)

I have been using two Large/X-Large duffle bags ($15, Eddie Bauer
Outlet) for family travel.
Campmor has large Columbia, wheeled, 6000 ci duffels on sale for $40
($140 retail).
Now that the kids are big enough, I have decided that they can pull
their own weight on trips, as well as the luggage across the ramp,
through the hotel lobby, etc.