I think it all depends on the kid. Some are very conscious of where they and their stuff are while others just leave them where they drop them. I had to travel frequently with much younger cousins, schlepping them to and from our grandparents, sometimes with stops in between. Overall, the easiest way was to allocate each kid with a lightweight backpack for their toys and activity books and then a large packing cube or two for their clothes that they would be responsible for packing themselves to be handed to me to be put in the large suitcase to check. I also found that the small and medium packing cubes to be great for each kid for shorter trips. Each kid got their own colour and the same number of cubes. Everything had to fit in or it didn't go. This also helped with squabbling (both the kids and their parents) over who could bring what.

I found that the younger kids were more conscious of their toys than of their clothes (jackets etc) and were more likely to keep track of them and carry it themselves for most of the trip.