There's been some discussion on the internet with regards kids learning The Flipper. It seems that several people have come across advice that warns against introducing the variation to children. Even today I've been reading a cricket novel that Joe's reading and in the Glossary it makes the point that younger cricket players should not try and learn the Flipper because of the stress it causes the wrist and the arm ligaments. My own personal experience is that it can cause problems with the joints in the thumb and I can also report that there's stresses put on the wrist, but I've only had problems with the Grimmett variation which does seem to require a lot more thumb leverage as opposed to the Warne/Benuad version which is far more finger driven.
I've done searches on the internet looking for medical advice or records regarding the issue and drawn a blank. I did come across a medical website that had studies into cricket injuries that looked promising but wasn't able to access the information. I'll keep an eye out for more info and I'll email Liz Ward on the www.bigcricket.com and see if she can shed some light on the matter as this is her area of expertise.