If your child knocks a tooth loose, put cold water ona piece of gauze and apply it to the site. Call the dentist (that's whom an ER doctor is probably going to call herself if you head there first). If it’s after office hours, head to the ER for an x-ray to determine the extent of the damage. Assuming it's a baby tooth that's affected, the dentist is likely to pull it if it's dangling. You don't want your child to inhale a tooth that's been knocked loose, but other than that, it's usually more of a cosmetic issue. Another reason to call your D.D.S.: If the tooth gets shoved into the gum, it might damage the developing adult tooth, and the dentist will need to treat that, too. The loss of a baby tooth may give the child a temporary lisp but won’t have a permanent effect on speech development.
If a baby tooth is whacked completely clear of the mouth, there's no need to save it. But if a bigger kid knocks out a permanent chomper, put it in a cup of milk and bring it and your child to the dentist immediately; he may be able to reimplant it if you get there quickly enough. The only (very rare) exception to the ER rule: If you can't find the tooth, and your kid is wheezing, coughing, or can't breathe, call 911 because it could be lodged in her lung.