Toddlers! Share Your Gameplay And Discoveries (New Thread In General Discussion)

If you have Better Exceptions by TwistedMexi, you can trigger an example exception whilst your toddler is selected. Under "Sim Info" in the exception report (which pops up on your internet browser), there's a traits section which mentions the quirks they have, even if they haven't popped up yet. So it does seem like the quirks are pre-chosen, before you actively discover them in-game.