Since the probability is of that happening is so small, I wonder if it's statistically significant. Since it's much more probably that someone's child will be a top 100 national junior, you'll perhaps get a much more accurate information on a the skills coach if he / she has repeated this feat 20 times, rather than making 1 grand slam champ by luck.
You can count out ITF Bollietieri because of the sheer number of talented students that go through that place.
Here is one guy who claims massive success (I'd like to validate it somehow):
"Over the past 10 years Heath has developed and coached 10 professional players of which his players have encompassed over 80 professional titles under Heath's tutelage. In the 7 year span prior to coaching the past 13 years on tour Heath's junior students won over 30 National and International Junior titles. Heath and Lindsay are also creators of one of the internet's most popular tennis instructional websites
www.virtualtennisacademy.com. Heath is currently mentoring many coaches throughout the world. Heath has been a key note speaker at PTR, written articles for the WTA, Tennisone and other publications, and enjoys learning something new each and every day."
http://www.tennisdev.com/aboutACE.html