Complex sentence: Cagney was the second of seven children, two of whom died within months of birth; he himself was very sick as a young child, so much so that his mother feared he would die before he could be baptized.
Split into simpler sentences: Cagney was the second of seven children, two of whom died within months of birth. He was sickly as a young child--so much so that his mother feared he would die before he could be baptized.