I regularly pack for my wife and two toddlers (1 and 3yo) in a 40L + additional 10L bags. It is very hard, means adults also have to surrender even more items to make room for books, toys and nappies, but it is not impossible. The reasons that made us do this is we wanted to only travel by train (europe based).

It’s a highly freeing exercise that is hard but doable. Only works for vacations (a day to a month) though.