Welcome to the forum.
Honestly, I don’t think there is a way to reassure the ones who have have been indoctrinating themselves for decades.
To be clear, I would be considered an american evangelical, and my background is in traditional Liberal Arts with concentration in language and literature, and then later critical theory, education and librarianship. As people fixate on science and Dawkins, they are forgetting that everyone’s basic assumptions will be challenged in every area of learning, if they are indeed learning. Being confronted with new ideas, no matter how neutrally they are presented, is hard. Profs are there to teach their subject matter, not tell kids that they came to the university fully formed, needing only a bit of polish.
The profs I had were ambivalent toward religion, at least in class. However, none of my fellow students ever attempted to highjack a class period to attempt to make it an evangelistic event or argument about faith. A prof might demonstrate antipathy toward the student, the argument, the content, the believe, whatever, if that were the case. Fellow students have the right to protest that a classmate is wasting their time and money as well.
If the grandparents you have in mind have been immursed in not only church itself, but also “evangelical culture” (popular books, christian radio, focus on the family, christian movies (think “God is not Dead”) christian publishing, podcasts, magazines, etc.), then no. They will not believe anything you tell them. How could you possibly know anything about the matter?
I spent much longer than average in state universities after going to public schools in the 1970s and ‘80s. I have a fair amount of experience. “But things have changed since you were last there in ‘02. It’s not the same.” I hear this from people with similar backgrounds to mine, but who have immursed themselves in evangelical culture.
The paranoia is real and then confirmed when college kids are pushy about their faith, believing they have some sort of authority to demand people engage in conversations that they don’t want to have, and then find those people can defend their own positions better, when pushed.
There is a lot more to say about all of this. You probably know that, or you wouldn’t have posed the OP.
A reasonable strategy for talking to paranoid family members is to remind them to pray for the young people’s spiritual development, that they would find a good church or Bible study, where they can grow in and into their faith; that they would approach their studies with real academic vigor and really understand the material, so they can consider well, what it means to and for them; that they would apply themselves with dedication on order to really earn the degree that costs so much money and work to get; that they will be patient in thinking through serious questions that confront them, being willing to chew on them for years, if needed; that they will be humble, faithful servants of Christ, demonstrating real love for the people they encounter and strongly disagree with.
College/university should be a challenging time. No one matures and grows when all their childish thoughts and assumptions are affirmed, and when the “pride of youth” is never challenged with the unexpected, unconsidered, unknown.