That's some adorable crap, there, Tayken.
I think that myth is probably propagated by sexist parents raising sexist children.
Any kid who tells you that boys should be with dad and girls should be with mom, is probably being told this by an adult who either mistakenly believes it, or thinks it will give them a custodial advantage.
As long as you are a good role model for a decent human being, you can raise a decent human being.