Be prepared to spend time relating to children before trying to photographing them in their environment. Attention spans are short, and moods and expressions change rapidly. The best opportunities may arise when your subjects forget about you altogether and begin playing in an uninhibited manner.
When working outdoors try using a medium zoom lens, perhaps 70 - 210mm, that lets you keep your distance but still fill the frame with interest. Indoors where space is restricted, something shorter such as 35 - 70mm is more appropriate. From a high viewpoint small children appear diminutive, so this is very much an adult's perspective. Better opportunities may be obtained by getting down to the children's level. The floor is a good place to start, although the occasional sticky finger in the middle of a lens may have to be accepted.
Teenagers are likely to be self-conscious and awkward in front of a camera, but keen to project a particular image. This can be a problem, so try working with groups of friends. The individuals draw confidence from each other, and eventually become more relaxed, so may be easier to handle than when they are alone. Be prepared to adopt their collective mood and take an interest in whatever they are doing.