Poor kid.
As was pointed out, the black eye could be completely gone by then.
If it isn't, I'm tempted to say that using concealer/cover up isn't really necessary, because you want your pictures to be real, right - to show who your daughter really is in this moment? And real life is messy and sometimes gives you a black eye. (Ouch.) I'm sure she's adorable, no matter what.
But if it really bugs you, and/or to make your daughter feel better, you can do a few things before/during your trip to help that bruise heal quicker. For one, since it's been a few days, you can try warm compresses (a few cotton balls soaked in warm water would work, hold one to the bruised area until it feels cool, then swap for fresh/warm one) to help get fresh blood circulating through the area, which can break up the bruise a bit faster. (You want cold ice on a fresh black eye to stop any swelling and help with clotting, but after a few days, it's okay to switch to warm.) You can also gently massage the area around the bruise - so over her cheekbone, to the side near her temple, on her forehead along/above her eyebrow, etc. Again, that increases blood flow, so will help the bruise break up faster.
It's also a good idea to drink a little extra water, and eat fruits and veggies, healthy oils, and healthy protein - give the body the right fuel to help it heal as swiftly as possible, right? (When my son had a nasty bruise, that was suggested by our pediatrician, who knows my son is typically a good/healthy eater. I'm not sure if what she eats will truly speed up healing, but I'd imagine keeping an overall healthy body couldn't hurt!) Like ktlm, I've heard that Vitamin C can help, but I'm not sure if it's true. Can't hurt to have an orange (or pineapple?) or a glass of juice though, just in case it does make a difference!
Good luck! (And really, if it is still visible, don't let it throw you for a loop. Enjoy Disney World with your daughter, and know that down the line, you'll be able to look back at photos and giggle together over her post-MMA-fight look!)
