I wouldn't say a word. It is a strange situation really, as their decision about whether or not to have children will have such an impact on your life, but it is totally their own business. You know your own children and if you have the slightest suspicion that they would find your questions intrusive, I would say nothing, as there may be good reasons why they choose not to talk about it.
We had difficulty conceiving our first child and had fertility treatment, which was still in its infancy
in those days. We kept our problems to ourselves and didn't tell our parents. It was stressful enough the two of us waiting to see if I started a period every month without having four other people involved - it would have been too much pressure. It took its toll on us emotionally, and also financially, as fertility treatment was not available on the NHS then and we had pay for private treatment in Harley Street which we could barely afford. So I was not very happy when my DM (who had two GC from my younger sibling who all lived in Switzerland) told me she thought we were being very selfish in remaining childless as she and my DF wanted more grandchildren.
I did forgive her ....... eventually!