whose is the possessive form of who, so the question is actually to whom the child belongs (the answer can be "Rina", "Gila", "David" and so on)
Except that we english speakers just say "whose." As much as it bothers you, that's modern English and we can't change it. Here, have a consolidation hug (^-^)/
If ?של מי הילד הזה translates to "Whose is this boy?", how would one say "Whose boy is this?" in Hebrew? Would it be:
I ?הילד של מי זה?