Renewed upon a dream has one of the easiest necklace tutorials ever. All you need is a hex nut and some thin yarn. She suggests using a needle to wrap the hex nut but little fingers should be nimble enough to wrap the yarn.
Good for ages 8 and up.
Get the instructions here:
http://www.reneweduponadream.com/2012/05/color-block-hex-nut-thread-necklace.html