My 3 yo daughter puts her hands in her mouth when she is anxious or nervous. We do remind her to not do it, but one thing her teacher found that is working is that she needs to stop what she is doing if she puts her hands in her mouth and she needs to go wash her hands. She does NOT like stopping the fun activities to constantly wash her hands. We have see a lot of improvement in the last month and now we can usually stop the hands from going in the mouth by warning her that she will need to go wash her hands. Also, her teachers (and us) offer alterate means of stress relief...a squishy ball, silly putty, etc. Good luck!