I have MANY first aid kits. I make them all myself. When the kids were young & we went camping, I bought the largest rubber-maid food container that sealed water tight, that i could find at w-world. Filled it with stuff that I/we actually have & would use. I got my first aid training in the U.S. Army. Not an expert by no means, but have never came across a situation where I could not help at all, even bad car accidents---unless they were already DOA. I also use the clear plastic w/rubber seal containers of various sizes you can find at any outdoor store or sporting goods section. Those are great for smaller ones, like back pack's & duffel bags & auto's.
GREAT things to have:
HIGH QUALITY precision tweezers
magnifying glass
quality fingernail clippers
waterproof stretchy tape
waterproof white tape
eye flush cup & solution
Tecnu-poison oak/ivy cleanser
Calamine lotion
Large/Jumbo band-aids
regular size bad-aids
square gauze pads
Leatherman Micro tool with scissors and knife that is RAZOR sharp. Being razor sharp is important if you have to do any type of surgery.
reg Leatherman tool with good pliers
91% rubbing alchohol
Hydrogen peroxide
foot padding for blisters
finger splint
triple anti-bacterial ointment
iodine solution
& the list goes on & on. I can tell you that I have used just about everything in ours at one time or another, including surgery with the Leatherman & the scissors to fix a bleeding ingrown toenail on someone else's kid, that was in tears. I told him that he could go to the hospital with his dad & end their trip, or I could do what the hospital will do, only right here, with no anesthetic . I told him it would hurt, but only for a minute. He let me do it eventually & it was actually into his skin about 1/8-1/4". Never seen one that bad. Fixed him up, next morning, he was back to riding his dirt bike. Also a good idea to have a quality first-aid book, in a waterproof bag.
This is a good one:
https://www.amazon.com/NOLS-Wilderness-First-Aid-Library/dp/0811728641