C-mom, you should do a search on play publishers and find two or three which match the genre and age level of your daughter's play. (I suggest going to your local library and sitting down with the latest copy of the Writer's Guide.) Look for the company's submission guidelines (they're almost certainly on the Web somewhere) and follow them. If they choose to publish her play, the copyright will be reinforced by having the play published. They will also file the document (book, magazine, business catalog) containing the play with the copyright office in the course of doing business.
This way, if anyone buys her script from the publishing company, your daughter will get a percentage of the price. It wouldn't pay much, but it's better than a poke in the eye with a sharp stick. And it might be a step for her in her career as a writer.
Mind you, the copyright is already in effect once the play is written, published or not. DCarson is right that filing gives you legal leverage in case someone tries to use it without your permission or claim it as their own work but doesn't actually establish copyright. (It used to work that way back in the day, but no longer.)