Mexican law would determine if he's committed a crime of bigamy down there, this forum only addresses US issues.
He can apply to bring his children here (if they are unmarried). It matters not who their mother is.
There may be issues with him attempting such application with the second wife (particularly if the first wife wants to call attention to the fact that she is properly his spouse).
Note, he can wait a long time to get them here on such a status. It's running close to three years for Mexico for spouses and children under 21. Much, much longer for children over 21.