I would give him the meal and ask him to sit in one of the tables while I talk with my manager about calling the authorities, paying for the meal my self and checking that the kid won´t stray away while I am fixing things up with the manager
If the manager gave me any shit I would tell him to go fuck himself, grab the kid, pay the meal and call the authorities, as well as a lawyer to sue the heartless manager if he even dares to try and fire me, its an easy case and I know I will win no matter what
I know managers are jerks but no one is so heartless or blind to realize that the kid is asking for food for himself, not money as would happen if they had parents watching over them, if he managed to get inside alone to get just one happy meal it means he escaped from the bastards that are using him to ask for money (yes, that scene in "who wants to be a millionaire?" where the kids are forced to ask for money, that is absolutely true in my country)
I would follow up his case from the beginning to the end, and I would make sure he is not lost in the system (as often happens) if the parents "appear" I would make sure "child services" (or whichever agency takes care of cases like this) investigate all the way the conditions of why was the kid asking for free food in the street.
yes, this will take time, money and a really pain in the butt to do, but if all ends good I might have just saved the life of one person, and that is worth all that and more.