My norm would be that legal immediate family go together (especially if you're pooling any duty free allowances) and children who are officially minors never go separately, but I would never go through immigration and especially customs with anyone whom I do not know extremely well. If asked, I would minimize the relationship, perhaps by explaining that we're just colleagues who work for the same large employer. (When it comes to customs, how well do you really know that boyfriend/girlfriend?) It's not etiquette, it's avoiding legal repercussions to oneself.