When the child has physical education, or participate in after school sports, the duffle bag will be essential. The child can carry their sportswear in the duffle bag, and there will be a side pocket for deorderant to assure they can clean themselves up. The main pocket will hold everything else. This will be a much better option than to try to load everything in the backpack. The duffle bag is also easier to wash and dry when it needs to be laundered (usually with the sports clothes).
When the child leaves my custody, they will be able to take their duffle bag with them even if they never used it. At the very least, the child will be able to pack their clothes in the duffle bag. Whether they decide to use it or not is their choice. The duffle bag is theirs to keep.