I would say “It’ll happen when the time is right” that way they have to second guess whether they asked about a sensitive topic and will second guess asking that, because that’s such a hard topic for a lot of people. I would lean towards joking/almost crass, but I know that’s not what you want.
You could just be super honest and say that that’s a discussion between you and your husband and you’ll let them know when you are ready.
“I’m not sure, do you know something I don’t?” But in a light tone and big smile.
I joke with my mil every time she asks I push trying back further. She knows we're not sure we're doing 2