I don't understand this "protect the children" stuff at all. Unless the crossdresser wanting to attend the book launch is some sort of lecherous pedophiliac perv (and I'll assume you aren't Hermione ), then what is accomplished by sheltering children from yourselves? Children see all sorts of things in assorted films and television, so where is the harm in seeing a "normal" person wearing clothes normally associated with the other gender? I encounter young children all the time and, although they sometimes stare a bit longer than adults do, I don't believe I have ever traumatized a single one.
Besides, most people who go to these Harry Potter launches, which to my recollection seem to be scheduled at midnight, are usually attended by teens and adults.