With luck everybody here has a few. We were presented with an opportunity to use ours earlier today, and I thought I'd share a few very basic things we learned.
Have your kit divided into two sections. Emergency, and non-emergency. The emergency side should have only a few TRUE emergency supplies in it so when everybody is in a huge panic they don't get lost in junk like little specialized bandages, medicines, and such. I'm thinking all that's needed would be scissors, Big (5"X9" or so) absorbent pads to stop bleeding, Neosporin, some smaller non-stick pads for once bleeding slows, and a good quality roll of medical tape. DON'T overcomplicated it!
The non-emergency side could contain as much as you care to stock in each kit. Stuff like eye wash, finger splints, motrin and the like, band-aids, so on... Stuff that you won't be in a complete panic looking for.
Make sure however many kits you have, THEIR ALL THE SAME! Having half a dozen different kits causes HUGE confusion when blood is pouring out of somebody!
Everybody in your family has a good idea what's in there, and good first aid skills for using the stuff to.
Not just you.This all came up fresh as a subject for us today when I was cleaning a folding knife I keep very, very sharp and had a little slip. Cut right down the middle of my thumb, and went most of the way through it. I made it to the bathroom sink before soaking everything with blood, but couldn't leave the sink once there without making the house look like a murder scene. My daughter was the only one there at the time to help, so we learned a lot about how to be better prepared for this kind of thing! Like antibiotic ointment that expired in 2007
REALLY STINGS!!!
There's a lot more i could share, but am typing this handicapped.

Most important of all. If you haven't gone through your kits lately. DO IT! And make sure others in your family do it with you. You may have shown them things a year ago that are now long since forgotten.
Have a safe, happy holiday! Check those first aid kits over! 