I found this pn Carl's Guide. Hope it helps.
1 Turn on the cheat: Testingcheatsenabled true.
2 Than Shift+Click the sim with a hidden trait.
3 Choose the option: Edit Sim in Create a Sim.
4 Change the age of the sim: Toddler, Child, Teen, Young Adult, Adult or Elder.
5 A popup on the right side of the screen will give the following information: Younger Sims cannot select as many traits. The following traits have been removed:' followed by a selection of traits, hidden traits and Lifetime Rewards that has been removed from your sim.
Click the X button to go back to your sim without having a trait removed.
