Osprey.com has the daylite 26 liter travel backpack. It costs more than the above suggestions, but is a quality bag that should last a long time.
LLBean has the explorer pack designed for kids. It’s a school backpack, but I bet it would work with packing cubes for travel. (Packing cubes - I recommend Ebags brand, classic cubes in sizes medium, small, and slimline.). My only concern about the LLBean bag is whether the main compartment is deep enough to hold packing cubes. I would investigate this further with LLBean before buying.
The patagonia lightweight tote pack and the Appenzell can definitely hold an Ebags medium sized packing cube and more. These two bags basically have one large compartment and a couple of extra pockets. Makes dropping in the packing cubes easy. The Patagonia bag is the easiest to pack. My family and I have used both with no problems.
Patagonia also has the atom backpack. Not sure it would hold a medium packing cube, but it should hold small and slimline cubes. 20 liter. Good size for a child. You can remove the electronics sleeve.