Introduce the kid you're helping to the keyboard. Show the kid where all the letters are. Since they're not in alphabetical order, it might seem counter-intuitive to the kid.
Show the kid how to place each finger on the keyboard properly. For the left hand, the finger placement from left to right should be on the "A," "S," "D" and "F" keys with the thumb on the space bar. Finger placement from right to left for the right hand should be on the "J," "K," "L" and ";" keys, again with the thumb on the space bar.
Ask the kid to type things out slowly with their hands staying mainly on the row of keys in the previous step. Other letters should be typed by the finger that is closest to them. For example, a "Q" would be typed by the little finger on the left hand. Practice typing things slowly will help the kid get a feel for the proper way to type, which will increase typing speed later on.
Incorporate typing games into practice time to help make learning to type fun for the kid. Various websites offer typing games designed to increase speed. A few examples are FreeWorldGroup.com and LearningGamesforKids.com.
Continue having the kid practice typing each day. Some of the practice time should be simply typing out words as slowly as necessary for accuracy with finger placement. For the rest of the time, games can be played to make learning fun and also to increase practice time.