Princesses need tiaras! Maybe the new Duchess of Cambridge would loan you the one she wore at her wedding. Seriously, look at party goods stores, or go online. Every girl at the party should have one. Don't know about inviting boys to a princess/fairy party, though. I would probably make it an all-girl affair. And I'd invite a friend to help me "do" their makeup. How cool would that be?
Speaking of princess weddings...I have to say that Kate's dress was gorgeous (unlike Diana's, which I realized at the time was awful). Now, maybe, every bride won't think she has to wear a strapless gown and show cleavage. Very sweet, demure and elegant gown.