This is a wedding in a hotel between non-Catholics; no one ever expected that it would comply in ever respect with the Catholic understanding of marriage. The bridal couple called you because they were anxious that you would take offense. Assure them that while you do not enjoy listening to people attack the faith and would hope that no one would do that at a wedding where the parents of the bridal couple are Catholic, you certainly will not miss the wedding because someone with an ax to grind will be there.
Even if it were a Catholic wedding, when we go to an event as solemn and joyful as a wedding, we go with the understanding that it will be our work, more so even than in daily life, to keep the event as unsullied as possible from unwarranted strife. It is too bad if the minister does not get that, but certainly a nine-year-old can. If we are provoked, we either ignore the provocation or admonish the ones trying to provoke us that “this is neither the time nor the place,” and let the matter go. “Above all, let your love for one another be intense, because love covers a multitude of sins. Be hospitable to one another without complaining.” 1 Pet. 4:8-9
Additionally, I’d give my children these two passages to work from on that day:
“Love is patient, love is kind. It is not jealous, is not pompous, it is not inflated, it is not rude, it does not seek its own interests, it is not quick-tempered, it does not brood over injury, it does not rejoice over wrongdoing but rejoices with the truth. It bears all things, believes all things, hopes all things, endures all things.” 1 Cor. 13:4-7
and with regards to rejoicing in the truth
“Now who is going to harm you if you are enthusiastic for what is good? But even if you should suffer because of righteousness, blessed are you. Do not be afraid or terrified with fear of them, but sanctify Christ as Lord in your hearts. Always be ready to give an explanation to anyone who asks you for a reason for your hope, but do it with gentleness and reverence, keeping your conscience clear, so that, when you are maligned, those who defame your good conduct in Christ may themselves be put to shame. For it is better to suffer for doing good, if that be the will of God, than for doing evil.” 1 Pet. 3:13-17
There are two truths in operation here. 1) A valid matrimonial contract cannot exist between two of the baptized without it by that fact being a sacrament and 2) many non-Catholics do not appreciate that, but it is our duty to assume they are doing the best they can with the light they have, and perhaps are even doing better than we are with the truth they have been given to accept. Since we Catholics tell them we have the fullness of faith, it is all the more important that we live in the fullness of virtue, especially faith, hope, and love.
I would go, to be a support for the parents (the primary ones in the line of any such attack, after all), and prepare my children to defend the faith with the gentleness and reverence required, should they be called upon to do so. With the help of heaven, the minister will not really go through with something so rude, but if so, his ill-conceived faux pas can be weathered with a grace that will speak well for the Catholic faith. That is the best witness, I think, especially for the bride and groom.