"You are eating the boys' fries."
Translation:Tu manges les frites des garçons.
I understand why you are looking for "les frites". However, if the boys ordered fries and you didn't and you were eating their fries, I could say "You are eating the boys' fries" (regardless of the fact that you weren't eating all of them) . Translation Tu manges des frites des garcons. This makes both translations correct. No?
Somewhere along the line you may have been persuaded that using "les" means all of them. It does not. Break it down. Whose fries are you eating? The boys' fries. There is no partitive here. How do you say "the boys' fries"? Les frites des garçons. Compare: the boy's fries (les frites du garçon).