To the best of my knowledge, the Church has made no official statement on the issue. Generally speaking, such things are left to the prudential judgment of the parents. Considered from a prudential viewpoint, if a particular “pageant” could be considered a “cute baby contest,” such an event does not appear objectionable. If, on the other hand, the event sexualized children by requiring them to wear costumes and cosmetics inappropriate to their age and their natural innocence, that kind of event could well be morally problematic and something a parent could decide not to have his or her child participate in.
