I agree it's up to you, I also think it's up to you to remind your kids if they are not going to believe in Santa to be mindful of other kids who do.

I love things like Father Christmas, fairies and all those magical parts of life, he was real once and possibly still is, It's not something I know for sure
So when the time comes for my kids to ask and I am certain they already know the answer, I will say yes he is real, he just needs help to deliver all that Christmas cheer.