There is always at least one quiet, shy child at any party, I doubt anyone thinks she's being rude.
Just sit with her until she feels ok about joining in, the more it bothers you and the more you and other people try to persuade her the harder it will be for her to make that first move. Don't try bribing her in any way, just get her some food and a drink if she wants some, find somewhere to sit and take it at her pace. If people are bugging her about joining in just say something like she's happy just to watch for now but thanks for thinking of her.
There is no reason why parties should be her cup of tea, we've all got things we don't enjoy.