I will ENCOURAGE my child to NOT play with your child...*EDITED*
if they are sucking on their fingers or hands. YUCK! I know its a hard habit to kick but I'm not risking my kids getting all of those disgusting germs. It's just so gross. I know I can't protect my children from everything but I will try to guide them to other children if I see your little one sucking on their fingers...Just saying!
eta this just applies to kids 4 and older. I understand babies sucking on their fingers.
Edit again. I don't know why you guys are getting so bent out of shape about this. I don't tell my kids specifically not to play with that child because they are gross and I don't tell thumb suckers to stay away from my kid. For example if there is a girl with a pink bow on the play ground and a girl with a blue bow on the playground. If the girl with the blue bow is sucking her thumb I will say to my child "why don't you go see if the girl with the pink bow would like to play" I don't snatch them away if they are already playing with the kids. I don't make a huge deal about it but if I can just get them away from a drooly hand I will. Yes kids need germs to build their immune system but I think they get enough contact with germs just by going to school and play parks. They don't NEED to have some kid wiping their drool on them.