Interesting question
I have had different approaches with my two younger children. DD1 never believed in santa or the easter bunny because they simply didn't exist for her. And by the time we found each other she was 10, so too old for tooth fairy or any other fairies. So I never had to think about any of that, I concentrated on finding some family traditions we could have instead
DD2..I have always been very honest with her, but that's not because of any belief I have in general, it's about her personally. She did actually believe in fairies when she came, but not in a good way, they scared her a bit. So when it came up I just explained gently that they didn't exist. Her FC's had been big on santa and christmas...but santa scared her as well. After all she had been through, the idea of some strange man creeping around the house at night was very frightening and upsetting :( To be honest, she was 8, and that's generally the point a lot of kids work it out, but being emotionally and developmentally behind, she hadn't got there yet. So I was again, reassuring her that santa didn't exist, and it was me
For DS, he has the lot! Santa, tooth fairy, easter bunnies, and aliens as well. I find it very magical setting it all up, and he delights in it. Especially leaving the mince pies out. He almost forgot this christmas! I reminded him that santa needed food after the hard work of delivering presents, and would be sad if there wasn't any food left out for her 
I don't actually agree with the idea we have to be honest about Santa etc. Every child is different, and they will need a different approach. The only adoption issue off the top of my head I think is 'one size fits all' is tell them they are adopted as young as possible. Everything else, you've got to use your best judgement as the parent. I think the belief in magic is different from being misled about your own past and background, and I don't generally think that that will damage their trust in you. What about children who have missed quite a bit of a normal happy childhood? They need to fill in developmental gaps, and part of that I think is trying to develop their imagination and sense of 'things can be fun'. I think Santa is a good thing for many of them. On the other hand there are children who do not trust easy and may need complete honesty. Or children with attachment issues may need their parents to provide all their necessities (food e.g chocolate coins, often in stockings) and nice presents as well. Kids like DD2, who need a secure home, which they feel very safe in (as in, no strangers coming in in the middle of the night!) I think magic is a normal fun part of childhood, and I think as I said before it can be quite important for development, developing the imagination etc. Also, as an afterthought, if they were the only one in their class who knew santa wasn't real - that would be one more thing that would make them different from their friends, when they probably want to fit in
Hope that makes sense :)