I can understand you wanting to keep the magic alive for your daughter due to your own traditions.
Perhaps this family finds the idea of people believing in magical people such as Father Christmas or the Tooth Fairies rather strange? Not to say that one group of people are right, but every one is entitled to their own beliefs and the right to say what they are.
As a humanist, i teach my children that for as many people in the world, there are as many ways to believe things. They are free to choose what they would like to believe or not believe- whether its Father Christmas, God, muliple deities (eldest son is currently being amazed by ancient egyptian beliefs) or just in themselves. As long as what they speak about, as i do, is pre-fixed by 'I' or 'some people' or 'Christians' believe/dont believe.
If you do want to talk to the mother or parents - instead of being seeming to be confrontational (as you say you dont want to be), perhaps express that you feel that you dont quite know what to say to her - that you know she wasnt trying to spoil anothers fun, but you were a little suprised and maybe apprehensive at having to address this with your own daughter at what seems like such an early age for you.
It may even be an opportunity to understand how she feels about it and make a deeper connection with someone through tolerance and listening - and an opportunity for her to do the same with you.