You need to install the Returner mod from NRAAS.
The only info I follow is where someone asked how to completely stop them from getting an imaginary friend. They were told to change the chance to 0. So I change it to 1.
https://nraas.wikispaces.com/Members%27+Retuner+Settings#NoIF
The path you need to use is:
NRAAS > Returner > General > By Tunable XML > Sims3.Gameplay.ActorSystems > OccultImaginaryFriend
kChanceToGetImaginaryDollsOnChildBirth > 0
I think the default is 0.75 - you'll be able to see it when you get into the path. If it's 0.75 then change it to 1. But if it's 75 then change it to 100. It is just shifting the decimal point.
You can download Returner from their site too.