Unfortunately a high proportion of children damage one or more of their front teeth in accidents. You should see a dentist as soon as possible.
However if the tooth is knocked out then there are certain things that you can do to improve the chances of it being saved.
Gently try and push the tooth back into its socket (making sure it is the right way around). Get the child to bite on a handkerchief and go to the dentist immediately.
If you can’t replace the tooth, place it in milk and again see the dentist as soon as possible.