I prefer to have a minimum of two kids at all times. That way it's easier to spot one in time when that rare collectible pops up. The kids seem to have a knack of turning into chamaeleons then. I'll therefore try to have the first baby once the youngest-but-one kid turns 11 at the latest. Usually, though, unless I've been unlucky in my collectible-hunting and dam-building, one will already be on the way.