Sweetest Pea, I don't know what the personality elixir does exactly, but we can't change anyone's traits or LTW's, so if it does that, then I think it would be a no no.
From the rules:
Spouses/partners' traits must be taken as is. In other words, if you bring Grumpy old Gunther Goth into your household, his traits stay as is, as well as his lifetime wish. For heirs raised from birth where you have the option to pick traits and a Lifetime Wish, everything must be random. All infants born into your game must be given two random traits at birth. You must accept the first dice roll and you can only choose a Lifetime Wish from whatever the game offers you (no Mid-Life Crisis and no Change Lifetime Wish). This restriction is for heirs and spouses only. Spares can have any traits or Lifetime Wish that the player chooses past the infant stage.
** The only thing I'm not sure of is that if you get to pick spares traits starting at child, if you can use personality elixirs on them, or not.