My eldest started asking to learn to knit at about three, and I foolishly tried to show him. He has asked and we've tried again about once a year since, until he properly picked it up when he was about eight. I know there are other ways to get young children going with yarn - pompoms, finger knitting and knitting dollies - and we did make pompoms, but I didn't know about finger knitting then, and never quite got the point of the knitting dolly.
What I hadn't expected was the sheer frustration of the whole thing. We'd sit down with yarn and needles and I'd demonstrate: "Through the front door, around the back..." He'd start, go wrong, I'd say, "No, not like that" all nice and calm, but it seems the damage was done. Within minutes he'd be throwing it down and saying he couldn't do it, with me not able to understand why he wouldn't just try again and having a full-on frustration tantrum. Yes, I mean me.
So the next time he asked to learn, I would tell myself to adjust my expectations. Of course he wasn't going to be able to knit perfectly, he may never learn to knit at all, but that wouldn't be the end of the world, the point is to enjoy it. Even so, I didn't usually make it more than ten minutes before pronouncing through gritted teeth that I was "just going to make a cup of tea."
But both boys learnt in the end and between them they've produced a couple of scarves for their teddies, and they're sporadic, but keen, knitters. I still do emergency rows to get things back on track - once one of them managed to turn ten stitches into twenty in a single row! But I have learnt to relax about the whole thing, and try never to rip back their work if there is any possibility of saving it.