If your child knocks out a permanent tooth, that's an entirely different story. These can and should be reimplanted, a process that's successful if you act fast:
1. Rinse the tooth in water (don't scrub it)
2. Put it back in your child's mouth (or, if you can't, in a glass of milk).
3. Take your child and the tooth to the dentist or, if it's off-hours, the emergency room.