Do they open up with you? If yes, whenever they share and just say that you'll pray/support them in their journey. If no, I suggest not to mention anything even if your intentions are good. We are a "TTC couple" for 4 years and I don't like the feeling whenever there are people who voluntarily opens up the topic about us having a child. I know their intentions are good, but it feels like they are pressuring me. It's not a good feeling especially we are doing everything on our end (including workups).
Yes, I agree with City Oh, you don't need to encourage them because for sure, they're doing they're best to be able to conceive a child. Just offer them little help to lessen their stress and workload. For example, watching over their older kids, helping them with some tasks. Avoid giving suggestions like, "Why don't you consider IVF?" You can find out more tips on how to support a friend with infertility here: https://www.verywell.com/how-can-i-support-a-friend-with-infertility-1960033 :)
Sorry, but i was just wondering if they have a religion?