There is an interpretation that is consistent with the way many drug dealers behave (at least in many portrayals).
The normal process of selling a drug involves the dealers collecting the money. But they don't want to get caught in possession of any contraband. So they take the cash and then signal someone else, usually someone more innocent, to make the delivery to the punter. I'm sure this setup is also seen elsewhere in Breaking Bad.
The situation with the shooting of Combo at least partially parallels this. The actual bad guys don't want to get caught in the act of selling or committing acts of violence. So they get less suspicious-looking underlings to do the dirty work for them. It might also be that Combo would be less wary about being approached by a child, making the shooting easier for the child to do (he can get nearer to Combo without suspicion).