We talk a lot in photography about making sure that we get the "correct" exposure in camera. This is particularly important in portraiture, as pulling your exposure up or down significantly in processing can make their skin look a weird colour, and gives a harsher contrast which is generally unflattering and best avoided! .
Great skin happens first in camera, so for the most part when taking images of people, we want to make sure that our subject is currently exposed. The best way to to do that is to use your subjects skin as your metering point!
And here's how do it :)
1) Spot Meter.
Do this by choosing spot metering on your camera - this is usually shown as a small dot in a square. This will allow you to meter for just a small portion of the scene, in this case, the skin.
2) Expose for the brightest side.
Hover your metering point over your subject's skin, using the brightest side - exposing for the mid tones is best - so not a highlight and not a shadow. You can see below two areas where I would spot meter from to give you an idea.
3) Add or subtract exposure according to skin tone
You can't simply meter to 0 when using skin as your metering point - you need to take your subject skin tone into consideration. If your subject is fair skinned, then you will need to expose at around +1 to get the correct exposure. For medium skin, expose to around +1/2 and for dark skin, you are looking at around 0, and for very dark skin, -1. Basically, the lighter the skin, the more to the + side you go, the darker the skin, the more to the - minus side you go. You will need to chimp and adjust as necessary. If you are only shooting your own children, you soon get a feel for where you need to have your meter read for their skin tone.
4) Over Expose Slightly
Slight over exposure can smooth out skin and have it glow. Don't overdo it though, perhaps just by 1/3 of a stop. This is personal preference, and you may need to bring it down in processing - however, I always prefer to bring the exposure down a touch than bring it up.
And that's it, how and why you should expose for skin in camera!