If somebody asks when you're going to have a second child, just ask them when THEY'RE going to have a fourth (if they have three), a fifth (if they have four) etc.
If they respond by saying "Three is enough for me!" (or "We'd like a third"), give them an innocent-looking Paddington stare and ask if you should tell a friend (mutual if possible) who has four children that they suggested they'd had one too many.
When they hastily backtrack and (invariably) say "No, no - I said three is enough for ME, but people can have as many children as they like", they have made themselves look very foolish indeed. Say no more and just smile and walk away, point well and truly made.