What's not in the best interest of the child?
Not having access to both parents.
Your statement;
The reason visitation is separated from child support is because studies justified overthrowing the old "tender years" doctrine.
I've had both a competent attorney and a family law judge tell me that the State has determined that both parents should have contact with the child because it was in the best interest of the child to do so. Furthermore, a parents support for that child was a separate issue because you basically can't tell someone they must 'pay' in order to visit their own child.
That's why child support is a separate issue from visitation.
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It seems we agree on the child having both parents. Have I misunderstood your post?