Don't know that they're technically terms of endearment, but my husband calls our sons First-Born and Last-Born. Interestingly we've had a whole series of nicknames for Last-Born and virtually none for First-Born - a function of their very different personalities, perhaps. For example, David at one point was Davey Doodle, which became Dipsy Doodle, which was shortened to Dips. There are still others, but you get the idea. (And God forbid that David, at 19, should EVER stumble upon this site and realize how far I've spread the family idiosyncracies!)