Regardless, couldn't Texas just charge her with contempt and issue an arrest warrant, and then California would have to extradite her based on existing agreements between the states on law enforcement matters?

This only matters in Sane World. We are in Clown World however, where women get away with kidnapping children all the time. Especially in this case, where the father had his custody rights revoked, even though he's the only one, who's actually related to the children.