There is no rule as to this effect. It just doesn't happen. Exalted do not lose dots of Permanent Essence when they have children.
It could be a misunderstanding of something that is canonical: that Terrestrial Exalted women age much faster if they have children too quickly. More than one child every five years will cause them to shorten their lifetime down to that of a mortal, but I don't recall any hard numbers ever being given. Also, all Exalted have a term of a full 15-month long year rather than 9 months like mortals.
The stuff I quoted might be in MoEP: Dragon-Blooded, but I think some of it was actually from Dreams of the First Age. It's been years since I've specifically looked. The character idea looks really cool. Legendary Breeding is going to make both characters Very Big Deals in the Realm. The only other canonical Terrestrial with Legendary Breeding at the current time period is, if I remember correctly, the Scarlet Empress herself.