I don't think there needs to be a talk about it, the child knows, and perhaps she's holding back because she knows it upsets her mother. Sometimes things progress naturally as they should and a huge sit down talk would be weird.

As parents we need to let go. It sounds like it's all about the parent here and not the child. There are many fun things to do at Christmas that don't involve a belief in Santa.

Sounds like the perfect opportunity to think of something creative and new. Ask the daughter if there is other cool stuff she thinks she would like to do. She's at an age where it's normal not to believe.