I'm so sorry to hear that your daughter is suffering so I'm afraid I don't have too much advice for you but I'd really recommend reposting this as a separate post with a descriptive subject line (just click "post" at the top of the page when you're looking at the list of all posts). That way more people will see it.
From what you've described it very well could be IBS but she *needs* to get all the necessary tests to rule out anything else. Also check out Heather's 'IBS: The First Year' (most chapters available on this site by clicking 'IBS Books' to the left). There are a few sections in there about children and IBS, including, if I remember correctly, some info about diagnosis. Also be sure to do a search - there have definitely be some discussions before about children with IBS that might be helpful to you.
But yes, do try reposting this as a separate post. I hope you get the advice you're looking for!