No, please do not ask. It really isn't your business, sorry. What do you hope to gain by asking, anyway? You say you fear your DiL may be stalling. If you manage to establish this to be the case (though heaven knows how you'd couch your questions to confirm that suspicion!), what would your next step be? You can't seriously imagine you could have a cosy little chat with her to chivvy her along?
If they plan on having a family, you will find out in due course! If they have been trying without success, they would not welcome your questions, no matter how gently you ask.
Keep quiet, let them come to you if there's anything they want to tell you or talk about. I know it's hard, but it's time for you to stand back now - your son is making his life with his wife and it's not your place to involve yourself in such a personal and possibly sensitive area of their lives.