A good way to start teaching kids about knitting is to use a round knitting loom (found at most craft stores). Kids as young as 7 or 8 can learn to use a crochet hook to create simple projects like a square hotpad or scarf.
Lion Brand yarn has good instructions on their site for how to crochet, including a video – click here.
Friendship bracelets, belts, rings and other items can be made from braiding and it is generally easier for children to learn than crochet or knitting. There are many, many different types of braids and many different types, widths, color, textures of yarn/string/thread/wire to work with.