I always kiss my babies on the mouth. I know lots of people don't, but I don't see the big deal.
I aim for their lips with my closed mouth, and sometimes I get their top lip, bottom lip, or their open mouth and get slobbered on.
So with that background of my girls frequently being shown lip kissing, DD2 started kissing me about a month ago (9 1/2mths or so) Not sure if she's done it to anyone else because I'm the only one who kisses her lips.
She hasn't learnt to do it with her mouth closed, and basically lunges at me with her mouth open. From memory it took DD1 quite a while to learn to kiss with her mouth closed regardless of whether she was kissing my lips or cheek.