I have not been to Katsucon, but I've been to something similar in my hometown so I'll give my advice based on that.
As to what it is, it's basically a fan convention based around Japanese anime (shows)and manga (books).
There's alot of shopping you can do; everything from keychains to toys to Japanese snacks, especially Pocky (long pencil shaped cookies covered in chocolate)

At the one I went to, you could even buy leather armor pieces, wall scrolls, and actual Samurai swords. Be sure to bring plenty of cash...you'll need it.
Prepare yourself for TONS of people in costumes from shows you've probably never heard of. The vast majority of people who attend these conventions wear costumes (or as we call it, cosplay.) Most people go as characters from animes, but I've also seen people go as Nickelodeon or Disney characters, Doctor Who; one even went as Rainbow Bright. You don't have to wear a costume to go, but if you do, it adds so much more to the experience! Just be prepared that if you do wear a costume, especially of a popular character, people may ask to take your picture, take a picture with you, or may even want to hug you. (You can tell them no.

)
Looking at how old your daughter is and also who is on the guest list, she's more than likely wanting to go for the same reason I went...the voice actors. (My avatar is actually a pic of me with one of the voice actors I met at my local convention.) At most conventions, different voice actors and actresses from different animes will come to do Q&A panels, autographs sessions, pictures, etc. These are always loads of fun; every voice actor I've met has been EXTREMELY NICE and pics and autographs are free.
Seeing that a certain voice actor is going, I'll give another piece of advice... Remember the name Vic Mignogna! This guy is just about the most well known voice actor in English anime and a rather large fanbase to boot. He'll probably have a Q&A panel just on him and he even has a Star Trek panel at some conventions. Super nice guy if you get to meet him. Just be prepared for LOADS of squealing girls...
There are something events that are probably not age-appropriate for your daughter, but for the most part, these are late at night and in separate spaces, so you should be fine.
The best advice I can give if you really want to see what it's like is to search '
Katsucon' on
Youtube. Several people will post their videos from conventions on there and that will probably give you a better idea of what to expect than anything you may read on it.
That's probably some good starter info for you. If you have any questions, I'd be more than happy to answer them the best that I can.