In our country, breaking our laws. Hold them in juvenile detention just until they are 18 just as we would an American child that committed a crime.
Sending them back to where they came from originally-bypassing Mexico-is probably cheaper and no one can accuse la migra of abusing minors...
If it is true that they came to Mexico from someplace south of there, then we will be doing Mexico a favor-one we can remind them of when we need to.