Everyone thought Lelouch and Nunally were dead, in fact I think they mentioned in S1 that the Britannian government had said they died during the invasion.
I know it seems wierd for no one to get suspicious, but this is the geass world. Everyone thinks they're dead so the possiblity that these two kids are the dead prince is just OUT OF THIS WORLD! Plus, Lelouch was living a life as a commoner, so anyone would probably think "Why would lelouch be living as a commoner if he has royal blood? He cant be the prince then!"