I'm a liitle confused. If you and your daughter share a family voice plan, the additional blackberry plan would be 19.99, correct? What was it you added to your daughter's line?
On your question, see the article at the faq http://www.blackberryfaq.com/index.php/Main_Page
on using a blackberry with a generic data plan, when the site is back up. In some cases, users can use opera mini with a basic carrier's data service, without blackberry data service.