My DD11 has just started secondary school. She still believes in Father Christmas. If she'd asked me in recent year if he was real, I'd tell the truth, but she hasn't. She's not pretending for the gifts, she definitely believes.
DS10 is more sceptical and thinks things through more and I think he has significant doubts.
My AIBU is that would I be wrong to tell them he's not real? Purely on the basis that DD is at secondary school now and may find herself the lone believer. Better she finds out from me? Or not? Or are there other believing children at secondary school?