I will assume that the child will have physical education, or after school activities. If this is the case, then a duffle bag will hold all of their activity clothing, and hygiene supplies such as deorderant, and soap if they will shower. This will allow the child to not have to worry about stinking up their backpack which will likely not be as easy to wash as a duffle bag.
When the child leaves my custody, they will be able to keep the duffle bag. In addition, they can pack their clothes in the duffle bag which will make it easier for them to take their clothing with them.