Can't help you here Jazzns. I never had to face the situation. I did not grow up in a strict religious family. I never had to shake off a straight jacket strapped to my mind.
I was atheists from early high school. Never flaunted it and never denied it. I knew and I was comfortable. That is all that mattered.
Let the kids know, of course. If you teach them well they will make up their own minds, either way, as it should be.
As for the rest of the family? I don't know what to say. If it were me I'm thinking I would not make some big splash announcement about it. If the topic comes up then fine. Say my piece like it was nothing new.
Maybe a bit of attitude with "What? Is this some surprise to you? Really? Well it shouldn't be, you know. How about some more turkey?" Some people appear to be more accepting if they are presented with a fait accompli in an easy going normal course of business no big deal matter of fact way even if
you think it is a bombshell.
But then again, I never had to "fess up" within the family dynamic you are facing. I wish you a calm wind and smooth sailing.