When I was out in the field with Marines, they all had to know at least some first aid just in case I got myself blown up or something, despite the fact that they treated me like a bunch of nerds with a first edition comic. Everybody needs to know the basics. It doesn’t matter how old somebody is, there are things you can teach kids to help them take better care of themselves or even others. You just have to know your target audience.
I bet there are books or videos you can show them if you want. However, you don’t really need to bother with all that. It isn’t hard to talk to kids. You don’t need to dumb things down but you do have to explain stuff to kids in a way they can understand. You can’t actually see germs, and getting an infection is something that happens in the future. They can both be hard concepts to figure out as a little kid. But you can tell them that not keeping a cut clean could make them sick. Most kids have been sick, so that is a concept they understand, so that is something they can work with.
Another thing you need to remember is that kids often learn by watching, so give them the chance to see what first aid looks like. Let them see you taking basic care of yourself if you yourself get hurt so they can see the things that you need to do.
Even small kids can learn how to put antibiotic ointment or cortisone cream on themselves and how to open packages of adhesive bandages. You can teach them how to recognize certain things, like what a seizure or broken bone looks like. Or what having an asthma attack or allergic reaction might feel or look like if that is something that is a concern at your house.
Once you think they can handle it, you can tell them where the first aid kit is, what’s in it, and how to use the items in it. If anybody in your house has a medical condition, teach them what the signs and symptoms are, what problems look like, and what to do about them. You can also give them more responsibility for their own care. If anyone has severe allergies, you will need to teach them how to use an epi pen. If your kid uses a rescue inhaler, they need to know how and when to use it.
If you don’t think your kid is ready for all of that, it is fine. There are still a couple of things you can do. Making a list of helpful numbers and an emergency plan and posting it where your kid can see it is an excellent idea. But the most important thing you can teach a kid is what we consider an emergency, and how to call 911. They need to know their address and phone number to help the dispatcher if they are at home when they need to call.