Who is Ivan Margolius from Under a Cruel Star: A Life in Prague 1941-1968 and what is their importance?

Ivan Margolius, born 1947, is the only child of Rudolf Margolius and Heda Margolius Kovály. Too young to remember his father's arrest, Ivan is raised by Heda and spends many months with distant relatives in the country. He is usually described as too thin and often sickly, though his stints in the countryside cause him to become healthy.