It is a bug that doesn't bother me since it doesn't do anything anyways. For the accidental "romance".
However, once activated it can't be deactivated (as far as I know) so if it is bothering, which I understand, use this cheat code:
modifyrelationship Little Boy Dads Lover -100 LTR_Romance_main
Replace Little Boy and Dads Lover with the names of the Sim characters. (the order doesn't matter, could put Dads Lover first then Little Boy. You can set it to 0 too if you prefer. Keep in mind any number will add up to a maximum of 100, and any negative will subtract. So if you don't want it too low below 0, then just subtract a little bit.
With a "child" it's irrelevant anyways. The "romance" won't take affect until they are a "teen".
I never had the problem with a "child" though with romance. Interesting.
And yes, I use a simple mod now where this is much easier to fix since the mod puts all the codes in for you. That's what a mod does. ;)
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I honestly LMAOed typing in Little Boy and Dads Lover for the character name examples. :D :D :D