ok. well, we did the whole santa thing with the children when they were little. They didn't take any notice of it (or christmas!) for many years 
but the last few years they embraced santa and christmas.
Only thing is now - they are 11 and 12 and it is really time to put santa away. It is important to me that they function at an age appropriate level and don't, as far as possible, behave in ways that isolate them from their peers. Talking about santa as you approach your teenage years would certainly do this 
So last year I switched over to what would you like me to get you for christmas? and telling them that I would be getting them gifts, and just trying to not mention santa at all. they continued to talk santa. I explained that santa is a lovely fairy tale that we tell to little children, but really, parents buy the presents.
They appeared to accept this.
Fast forward to this year and it's santa this and santa that. Leaving mince pies out for santa, will santa this, will santa that.
And I am doing the whole "I will be buying your presents soon, write me a list and I'll choose from that" I had to have the whole santa doesn't exist conversation again. They nodded, but clearly refuse to accept it because they still talked about santa, including leaving a biscuit and a glass of milk out.
Can anyone help me to understand why I can't get through to them about this? Suggest ways to do it?
Keeping them believing in santa is not an option. I can't have 2 grown men getting excited about santa claus.
But I spent several years telling them all about santa and now I'm telling them that none of it was real and they cannot process this information.
I clearly made a huge, gigantic, massive cock up.