The same rule goes for sims with Daddy/Mommy traits: they need a low romance to start Sugar Relationship with Sim with Sugar Baby trait, but high romance to start with Sim without the trait.
Sugar Baby also can ask Sims without Daddy/Mommy traits, but they need to have a very high romantic relationship for that option to appear. For that, they need to have the bare minimum of a romantic relationship. Sims with Sugar Baby trait can ask Sims with Daddy or Mommy trait to became their Sugar Daddy or Mommy. This will help you to populate your world with Sugar Partners without much trouble. In the setting menu on computer you can find cheat menu where you can turn any Sim in the world into Sugar Baby, Sugar Daddy or Sugar Mommy. All traits are available for ages from Young Adult to Elder.
This mod adds three traits to the game which you can buy in Reward Store: Sugar Baby, Sugar Daddy, and Sugar Mommy.
