Well actign childish isn't all that hard..........but it's actually quite different to actign as if you have mental health problems..........as mentioned before, you have to have consistency and not turn out looking like a clown!
But it's different for different people!............Gurdeeps actign as Vedika was very loud.........running and screaming........
Whereas Sharad's acting as Sagar is most of the time small and quiet - exactly like a baccha.........
Personally I feel it would be easier to acta s Sagar than Vedika!