I've been teaching my kids.
My daughter is five. She sits on my lap holds the left needle by herself, and the right needle. Right now I throw for her.
I figure she needs to get the pattern and motion down.
My son is seven and I've almost got him doing this right. He's got it all down until he has to pull the new stitch back through the "window or door" whichever it is... so when "off jumps Jack" the new stitch falls out. Essentially he goes through all the motions and ends up slipping the stitch.
I was using regular sized needles with them, but ended up purchasing them small plastic needles. These work really well and the kids are happy with them.
I do about 5 mins a day and we stop the second they or I get frustrated.
It took about 5 days for my son to get to the point where he is.