First aid is not a substitute for trained medical care. It just tides you over until you can get to a professional.
But Murphy's Law and the continued presence of Dennis guarantee that, sooner or later you will push the envelope of common minor injuries - especially if you have children in the house (even as visitors). Think: splinters, something in your eye, falling off a ladder, slamming a door on your hand.
Wound dressing:
Cleanliness:
Medication:
In case of poisoning:
Misc:
Practicality argues that supplies in the car are subject to heat and cold that will reduce their shelf life, and do you really need a spare epinephrine pen in every car?
So, use a little good judgement, look over Home First Aid Kit, and pick and choose what goes into the car.
A minimum would probably be:
Just make sure that every car in your household does indeed carry emergency supplies.
Let's go for a walk! Or perhaps take a horseback ride down into Maui's Haleakala volcano. If it's an organized activity or paid tour, the organizers probably have some first aid stuff with them. But if you just take a hike with some friends, you should bring some supplies.
Here's the trick - if your kit is too big, heavy, or otherwise a nuisance, you will probably leave it home. So you have to carry the minimum amount that you think will be both convenient and helpful.
If you have a pack with you, you can get away with more supplies. But no matter how light you are traveling, you can probably afford something the size of a wallet.
It might contain: