This is not an easy task
By the 1851 census the Malet children all seem to be at boarding school, I see a couple at Eton. There is a Richard Mellet of the right sort of ahe as a pupil at a school in Oxford, but no way of telling his parentage from that.
Back in 1841 I tried to find George at home in Wiltshire to see if he had a little Richard with him, but no sign of George. I f Richard was born in India, perhaps they were still out there then.
Have you tried consular births? (I think on pay to view site findmypast)