Your advocacy extends beyond the particular child to the greater question of child baptism. As noble as that is you cannot prevail. The target is too big, too well entrenched and too well defended. From a strategic perspective you are not scoring points with the elder but adding to the body of evidence that you are an opposer. That is your particular label. You will not be shunned. That treatment is reserved for apostates and other baptised ones targetted for punishment, but you will be discounted, pitied and ignored. Your elder may continue to communicate with you but it is not because he wants to understand your perspective. He already knows you are wrong. It is to keep an eye on you or, better still, keep the lines of communication open so that he may possibly help you to understand his - aka the Watchtower's - perspective. To understand is to believe. What a wonderful outcome that would be, to bring you into the fold to join your wife and daughter.
I think so long as yours is a confrontational approach the probability that you will be able to penetrate the elder's armour will be near zero. A respectful, logical and long term approach would be more effective but the odds of success still very low.