Could you imagine explaining that's one to the kids? "Mummy? Why has Father Christmas got a lady's voice?"
Actually, my parents hired people to come to our house and play St. Nicholas when I was a kid.
One year, St Nicholas was a woman, with a beard glued on. I did not ask "Mummy, why has St. Nicholas got a lady's voice", for the same reasons I didn't ask "Mummy, why does St. Nicholas look so different from how he looked last year?" or "Mummy, how does St. Nicholas manage to bring presents to ALL the children in the world on ONE day"
It would have ruined the illusion. I was well aware that it was an illusion. I would have complained if St. Nicholas had suddenly apparead without the bishop's apparel but with Father Christmas outfit. That would have ruined it for me.
A female voice? No problem at all.
Children don't ask why Father Christmas has a young man's voice despite wearing white hair and beard, do they? Children with grandfathers are likely aware that, at a certain age, men's voices show their age.
Children are way more perceptive than many mumsnetters give them credit for.