There is no way you can determine this before you have them. Each child is different. I have two daughters who are highly intelligent and were reading at 4 yrs. The older one might have been able to knit at 3, the other would have been too impatient and immature. When he time comes you will know.
When asking questions ALWAYS post the name and a link for the pattern if you have it.
Patience and attention, even more than dexterity, I think are the keys. A kid with less dexterity but more patience will stick to it long enough for the skill to kick in and for him or her to actually make something.
Well, I wouldn't put so much importance on teaching a young child to knit. I would let the child decide for themselves if they seriously want to take the time to sit down and learn the craft rather than try to teach them when they are unwilling.
Then again, I don't plan on having kids, nor am I going to raise yours, so our views on child rearing may be respectfully different.
I would start with something a little less complicated just to test the waters. Doing this might help determine if the child is ready. My DD got one of those weaving projects for her 6th birthday. It basically requires one to stretch cloth over hooks as the warp and then pull them opposite on the weft. That doesn't require as much dexterity as knitting. However it does require a little patience. I can tell that after that project my DD is not ready to knit.
Or maybe exploring some of the concepts of knitting such as asking them to help choose yarn or looking at patterns together. I always liked doing that with my Mom when I was little.