It sounds like you need an explanation/justification from your children. Just, "I don't want to," is enough.
I know why my child doesn't want to have children. We have talked about it because she has chosen to share, we are close and have that kind of relationship, but I never felt I was involved in the decision or had a right to approve/disapprove of it. I have never had a sit down discussion with them where I needed them to demonstrate that their reasons were good enough for me. I've offered appropriate thoughts in a supportive way, but also hopefully to help their thinking process so they're really sure. I'd hate them to regret their decision but, even if they do in the end, that's their issue to deal with.
I'm pleased to see them leading their own life and doing well. Whether they add children to that or not has nothing to do with me.
I think it's okay to be disappointed not to be a grandmother if you are, you can't help how you feel anyway. It's not okay to put that disappointment on your children or pressure them to make such a huge life decision to satisfy our wishes. I don't appreciate it when parents do that to me over any other issue. I'm glad my children are doing what's right for them.
I'm living my own full life. If grandchildren came they'd be loved and welcome in it but grandchildren aren't a requirement for a fulfilling life. I hope my children continue to live and make decisions that are right for them and can continue to say no to anyone who pressures them in any other direction. In fact, if I were present and I heard someone trying to put them in a position where they had to defend their choice, I'd probably come over and shut it down by changing the topic.