Teaching children to knit is not for the faint hearted. I have gone through the motions with many children, some of them even related to me, and people ask me quite often for tips on needle types and yarns that are suitable for this purpose.
My standard reply to these questions, is that when teaching children to knit,I like to start with shortish, bamboo needles at about 4.5-5.5mm diameter, and a medium weight (8ply, 10ply) yarn composed of mostly wool. My reasons for this are that little hands cope better with short, not-too-thick needles, bamboo needles are not slippy or sticky (or particularly damaging when poked into someone's arm or leg), and that wool yarn, if selected judiciously, tends to feel soft, comes in great colours, and is less likely to split and slip than cotton, or stick like acrylic, and is relatively inexpensive.
I follow this advice myself, and have started several accomplished knitters for whom I like to take credit. I also have a 14 year old daughter whom has so far only started 3 scarves, of differing types. You can't convert everyone into a knitter :(.
My latest convert is my son. He has been trying knitting on and off since he was 4 years old. He has several partially completed 8ply woolen scarves in his WIP pile. Some of them are rather old.
Only 4 years after his first attempts, he has completed his first project. He did not follow any of my standard advice. Instead he followed the advice I generally reserve for older beginners - pick something you want to wear, and knit that.
He fancied the Sirdar giant balls of yarn marketed as the one hour scarf. This yarn is not only 100% Nylon, but has a chenille effect, so it is a bit tricky to see the stitches. It requires 10mm needles, and both knit and purl stitches. Much more complex than the standard garter stitch rectangles he had fiddled about with previously.
I think it was the colour and texture that appealed to him - don't you think he is sounding like a knitter?
This scarf took about 6 weeks of sporadic beginner knitting.
He has cast on another project already.