If  you ever have a tooth that gets knocked out hold the top of  it and not the root end.  Rinse it off but do not clean or handle the tooth more than you need to.  Re-insert the tooth in the socket it came out of using a clean piece of gauze or clean cloth.  If you can not re-insert it bring it with you to the pediatric dentist in a glass of milk or water.  Time is very important in this situation.  So get to the dentist as quickly as possible.