She was definetly out of line. How you handle your money is of no concern to anyone but you and your son, so she should not have commented on it.
Plus, did she even know for sure he was your son? I know you said you know all the tellers, but maybe she didnt know you as well as the others? What if it was your nephew, or maybe you were dating a much younger man? It could happen, and if she didn't know you, then she definetly should not have commented. If she does know for certainty he was your son, then I can see WHY she might have been inclined to say what she did, but she should have kept that comment to herself regardless.