I'm going to talk to the kids at the house at which I just delivered food.
Is there something in particular that you are going to say to the kids?
I'm going to give them a warning that their door is unlocked.
I understand for your concern about their door being unlocked, but it's not appropriate to talk to the kids about that. You shouldn't go out of your way to talk to children that you don't know. If you want to let someone know that they should lock the door, you should talk to an adult about it.
I'm on a mission to keep children safe.
Your desire to keep children safe is commendable, but interacting with children of strangers is a thing you should avoid doing. You don't want to make the children uncomfortable and you might alarm parents and any adults that take notice.