M.C.
I got my daughter to stop sucking her thumb by making a game out of it. I would just yank her thumb out of her mouth and say, "NO sucky!!" and she would laugh, sometimes responding by putting both thumbs in her mouth. So I would again yank both thumbs out at the same time and say "No two sucky!" and repeated it til she got tired of the game. Eventually, and not very long, she stopped sucking her thumb.
I was lucky it worked, but I didnt want to associate any negativity in stopping her. But she started sucking on her tongue, when tired, and she still does it at age 5. You can see her tongue going between her slightly parted lips. Definitely cute, and non damaging. But I hope she stops that by the time she's at least 10.