What a strange story! I really can't see what she stand to gain from lying about having a daughter or not and who that daughter is. I don't know how old the daughter is - maybe if it's a baby then it's hers' and babies sort of look alike?!
At any rate, I don't think there is a way to remove any info from someone else website \ myspace page, unless you have her user name and password (think about the opposite - you wouldn't want her to be able to change your page either). I do however think that you might have a few options in making her take off the photos.
It's more a matter of legality then a matter of technology, but I think a letter from a lawyer threatening to press charges if she don't remove them being she's basically lying might do the trick, if she only did this for spite, she probably wouldn't want to to get dragged into court for this.
If that doesn't work, I suggest talking to the lawyer about writing another letter to myspace - they are not really responsible for content but they do monitor people's pages and if you can prove that the girl in the photos is in fact your daughter, they might remove the photos.
The beauty of the internet is the freedom it allow people to put up photos and texts and express themselves as they see fit, sadly that also make lying and spreading rumors very easy.