fredthered is correct that there is no “dance program” to which she can apply.
What she can do is submit an Art Supplement:
http://admission.stanford.edu/application/freshman/arts.html
It is true that submitting such a supplement affects the application deadline for both REA and RD.
Regarding legacy advantage–to my knowledge, it’s there whether your daughter applies early or regular.
She certainly has competitive academic stats (i.e., if she gets rejected, then it won’t be because of her stats). Therefore, I would recommend applying early with an Art Supplement if she prefers to get a decision from Stanford by mid-December.
Her unusual excellence in and passion for dance will certainly be a nice “hook.” Stanford undoubtedly loves people who are excellent at things, but what it loves even more are people who are genuinely passionate about the things they do (i.e., they aren’t doing an activity simply to get into college).