Awww, it's sweet he wants to 'mother them' like that, I guess he feels it's his job to keep the kids safe from the great unknown on the other side of the gate.
I agree, though you don't want him to use his mouth for this. The squirt bottle works real well in sittuations like this. After a few squirts accompained by a good firm "NO" you'll just have to say no and show him the squirt bottle, then soon you can eliminate the bottle, and just say no.
I doubt it will take very long, just be consistant, He's still a baby himself.