What you state is not official policy. Official policy is that there will only be separation in that case if it is doubted that the child is with the legal custodian…for obvious reasons.
There may be some anomalies, and some groups will doubtless contest that the policy is followed.

"In a White House press briefing Monday, Nielsen said, “DHS is not separating families legitimately seeking asylum at ports of entry.” But she said DHS “will only separate a family if we cannot determine there is a familial relationship, if child is at risk with the parent or legal guardian, or if the parent or legal guardian is referred for prosecution.”

Burnett also has reported that some families are not being allowed to request asylum — that they are being repeatedly turned away and told the CBP facility is too full to accept them.

Nielsen has denied that some asylum-seekers who present themselves at a port of entry are being turned away, which would be a violation of international law.

“We are saying we want to take care of you in the right way. Right now we do not have the resources at this particular moment in time. Come back,” she said."