C.P.
I know this is an old question, but since I am having a similar issue with my 15 month old son, I wanted to find out if this was something that babies do and I found this. My son has been doing this since he was about 12 months. He has never wanted a paci, has been a thumb sucker since nearly day one, and as he sucks his thumb he puts his finger in his belly button. Its so funny to watch, but it has evolved into sucking his thumb and sticking the other finger in another warm place...either into his diaper from the top or side (depending on what type of clothes he is wearing) or he will stick his whole hand into the BACK of his diaper and walk around the house. He has many times reached into a diaper of poo. Its frustrating. When I catch him doing it, I tell him, "get your hand out of your pants" and he does it without complaint, but usually back at it after a moment or two. If I am holding him, he will suck his thumb and try to stick his hand down my cleavage. With him its any warm, soft place. I don't get it, but it makes him happy. I am okay with the belly button, but when he does the other places, especially in public, its hard for me. I'm always like, "Oh please don't do this here.." when I see him going for the butt crack. I am curious if any other little ones have the obsession with putting their hands in a warm place while sucking their thumb for comfort. I gotta say though, when I sneek in and see him sucking his thumb with his hand in his pants while he naps, I find it hilarious!