prasad,
I will fully agree with you on Bhavatharini. That Raja gave her songs because she was the daughter. But in case of Yuvan, and I have been arguing this on twitter as well, Raja has kept him at a safe distance. He has only called him when some 'fun' songs were needed, as in 'Kadhal Jaathi'. In those cases, because the voices were supposed to be bad and Yuvan's voice fitted that to the 'T'. Can you give me one example where for a nice melody Raja has used Yuvan earlier? So I think it is very unfair to level this charge against Raja in the case of Yuvan, while it is perfectly fine in the case of Bhavatharini.
My personal take is, that using Yuvan would have been a decision taken upon GVM's request. For all the scorn shown, Yuvan seems to have his own set of youth fans. Now that makes no sense to me but the youth of today seem to have a predilection to bad voices and bad accent. Honestly do you think Udit Narayan or Sadna Sargam would have sung so many songs in Tamil had the youth been intolerant to the massacre of the language. As I said earlier, it seems to have been some sort of strategy to attract the youth and hence Yuvan got to sing the song. Maybe they think this will also become a hit like the 'oru kal oru kannadi' song from 'SMS'.
So before we throw stones at Raja, which we love to do at every occasion, and compare him unfavorably with others, try and find out why this happened, especially given the track record of Raja never using Yuvan for such songs earlier. That's the least we can do to Raja rather than come up with accusation of nepotism.