Osgood Schlatter's disease is a very common presentation in children, often during a growth spurt, where an imbalance exists between bone and muscle strength.

I do not feel it would be appropriate to use any electro physical agents at all, especially as you would be applying it over a growth area of young bone.

Therapeutic assistance to relieve pain and the load on the insertion would be appropriate. This could include taping to reduce the load on the tendon and perhaps some general lower extremity strengthening.

Mechanics are probably less of a factor. Kids are kids and some get this "disease" albeit for little more than 12 months. Check their shoes and diet, modify their training intensity and duration for the next 12 months and let their body rectify itself naturally. Using things to reduce the pain will simply result in nice bilateral calcifications in the tendons for the rest of their life. They must understand the process and modify their daily behavious accordingly.