How to stop a toddler from touching Mom's boobs
Personally, this never bothered me. I just deal with it because I know it's comforting to them and I know they will grow out of it. If it does bother you though, I'd suggest redirection. Pull their hand away from your chest and play a hand clap game or something. Watch for patterns - do they do it when they are upset? Bored? Sad? Try to avoid or figure out the root cause for the issues that lead them to touch.