In a sane world we would proceed as though there is a parent somewhere in Mexico, or one of its southern neighbors, who is looking for their child.
It would seem proper to deliver the 80% to the U.S. offices of Mexio's government, or that of the child's native country, for proper return.
Those governments are better equipped than the U.S. to resolve such family matters. Such a policy would have the additional result of Mexico more strongly enforcing its borders thereby reducing the damage to the U.S.
Rhetorical question: "What happens when a country refuses to enforce its borders?"
Knowing how prevalent human trafficking is, especially the worst kind that preys on children, another question is “What happens when SOME people in a country disregard child sex slavery for their own political purposes?”
The worst possible answer is that there are those that are fine with the added Democrat voters (illegal or not), the cheap labor and the pedophilia.