If you haven't already, you must listen to Mere Paa from Paa. It takes a while to recognize the voice, if you don't already know that it's been sung by Amitabh Bachchan. Of course, he's been singing a lot of his own songs throughout his career, but this is in a league of its own. It captures the innocence of a child with in the ageing voice of the character whose physical growth is accelerated but mental growth is normal. Absolutely amazing.
By the way, can somebody remind me of the name of the Robin Williams movie with a similar storyline?
While we're talking about songs, if you haven't heard the songs in the soundtrack of Couples Retreat, it has a few tracks by Rahman which are rather different from his usual stuff. One of them is in Tamil. This one is in English and Hindi.