What happens if a child swallows a loose tooth?

Nothing: It comes out of the other end. Nothing to worry.

Usually nothing.: Unless a child were to choke on it and inhale the tooth into the lungs, the tooth usually will pass easily through the digestive tract and be passed out with the stools.

It's OK: It will "work out all right in the end." you may, however, have to leave a note for the tooth fairy.

Stomach acid: Are you sure the child swallowed it? Most likely it will dissolve in his stomach acid. Don't worry.

Passing time: The stomach acid won't do too a heap of damage if you wanted to sift for treasure assuming the tooth holds sentimental or religious value. Otherwise fruits and veggies and plenty of fluids will assure its safe passage. I hope this was helpful.