OK, I'm going to share something else I do that may seem a little over-the-top, but (again) if I have it I will never need it!
Using a graphics program on my computer, I create a little "baseball card"-type profile of my children -- a small, close-cropped headshot, height, weight, hair and eye color, birthdate. All four kids fit on a half-page that I fold up in my wallet.
With four kids, I've always worried that in a lost-child or medical emergency, I'll panic and forget how much my lightweight DD8 weighs, or how tall DS5 has gotten.
I really only carry this on trips, but I also figure it's a way to ID that the kids are mine (not so important now as when they were pre-verbal). DH carries one also on trips.
You could also do the same by writing the info on the back of wallet-size school pictures.
We also use the buddy system -- DH and I are buddies with the younger two, and my older two are buddies.
PHXscuba