Conversation so far:
What were Robert Stewart, Viscount Castlereagh's parents like?
Robert Stewart, Viscount Castlereagh's father, the Robert Stewart senior, was an Irish politician and prominent Ulster landowner.
Who was Robert Stewart, Viscount Castlereagh's mother?
Robert Stewart, Viscount Castlereagh's mother, who died in childbirth when he was a year old, was Lady Sarah Frances Seymour-Conway, daughter of Francis Seymour-Conway, 1st Marquess of Hertford
Did Robert Stewart, Viscount Castlereagh have other siblings?
Charles Vane, 3rd Marquess of Londonderry's half-brother from his father's first marriage was Robert Stewart, Viscount Castlereagh.

Question: How many passed as children?
Self-contained rewrite: How many of Robert Stewart senior's chidlren passed as children to adulthood?