As lot of people have pointed out - there's not one thing which defines the 'it' factor. But in any sport the talented ones show glimpses of ability which is better than the averge kid in a group.
So if your kid is better than the average kid, then I feel they have some of 'it' factor. They have given you something to work with.
Look at all the doubles player who make a living on the tour making more than million dollars a year, I am sure not of all them were freaks as kids.
Some persisted longer than others, got the right break through and they are making a living. I am positive there are lot of singles players who are probably
better than the world class doubles player but struggling to stay in the tour.
In a interview I read Carlos Moya hit with Rafa when he was 12 years old and did not think he was going to be that great. So if Moya could not predict
about Rafa at 12 years of age you think anyone can predict how good a kid can be.
I have to say, tennis has to be one of the wierd sport were people discuss so much about succeeding and talent when the kids are 8 years old.
You don't need to be a freak at 8 years of age to become extremely good at 14 or 18 years of age.
In conclusion if you kid is keen do the best you can, don't put any upper limits on what they can achieve
and I am sure like any other sport you will find out how far he/she can go.