babytinkabell the fc I have at the moment came to us last September. They are 12, 7 and 5. They had never had a proper Christmas as in, no tree, stockings, special food or family traditions. They got a few cheap presents from extended family but that was it.
I was like you, really worried about overwelming them. In the event they loved it. We did all the normal stuff like snacks for santa and reindeer, glitter footprints on the stairs, lots of card and decoration making.
The younger 2 did ask why Santa had never come to their house 
. I had no choice but to make up a story that mummies need to write and tell Santa how many children were in a house so that he knew what presents to bring. I said that the letter must have been lost (rubbish I know but I was really on the spot).
They were in a constant state of excitment all through December with pantomimes, visits to Santa ect. I will never forget the wonder on their faces when they found their stockings.
Anyway I would say do what is normal tradition in your family. Maybe you could explain that all families are differant and have differant traditions. Ask her if there is anything special she would like to add to the festivities.
I bet she has a wonderful time, so will you. That's what make fostering so great. Giving children special memories that they can use themselves one day with their own families.