It is hard to hear them asking about this especially when you would like nothing more than to comply! Since youa are still ttc, I would personally say something like - yes, we also hope to have another baby but it might take a while (or a long time - since a week can be a long time to a preschooler!) and sometimes it just doesn't happen.
Dd is 4 and asked about this again last week after starting reception and seeing all the other siblings coming to achool. I told her I also wished we could have another baby and we did try but the doctors have told me it isn't possible. I did let her know I was sad about that - because it's the truth. But then I don't won't to go on and on about it either.
Dd also talks a lot about having a baby of her own - maybe when she's 6!