I think the best approach is to put a child in a position to succeed when
they decide to reach their full potential in tennis. This requires more than just refining technique and hitting endless balls as a youngster (there is indeed a law of diminishing returns as their strength is not what it will be, and their technique cannot be perfected quite yet due to this). One needs a strategy from day 1 to build a better athlete, because today more than ever, you need to be an exceptional athlete to be a money making pro. Introducing kids to different sports, light plyo's, multilateral movement, balance, and agility, are ways to ensure that optimum athletic ability is achieved when they begin more intensive training (their nervous system is "plastic" at a young age). But, even here, there are no guarantees. Who would have predicted from the Nadal/Gasquet YouTube Video (Competing at 13 years age) that Nadal would end up as a freak athlete. There certainly wasn't a ton of indication from the video: http://www.youtube.com/watch?v=KzKuv4j67aw
For Nadal, it's quite evident that puberty transformed his body and athleticism. But, "puberty" is not created equal for all boys. This is where genetics comes into play. Some boys are literally transformed into different looking creatures throughout puberty, while other boys do not gain quite the same physical advantages.
Here's a snippet from an interesting article about specialization vs all-around sport engagement:
The third study, conducted by Carlson (1, analyzed the training background and development patterns of elite Swedish tennis players who were very successful in international competition. The subjects were divided into a study group that consisted of elite adult tennis players and a control group that was matched by age, gender, and junior rankings. The most relevant findings are shown in the summary of research on page 35. Both groups of players were equal in skills up to the age group of 12 to 14; the difference in skills between the two groups occurred after this age. Additional findings in the control group were that skill development was fast during early adolescence and these players participated in an atmosphere of high demand for success. Interestingly, the control group players specialized at age 11, whereas the study group did not begin to specialize until the age of 14. In fact, the study group participated in a wide variety of sporting activities during early adolescence, whereas the control group performed specialized, professional-like training. Although the control group demonstrated significantly greater performances as juniors, the study group demonstrated their highest levels of performance as senior athletes. The work of Carlson (1 supports the importance of a multilateral training approach that is marked by all-around sport engagement and less professional-type training during early childhood and adolescence.