I wouldn't tell her. Ask your parents if they want her to know. If they do then you and they can decide how to tell her.
It is nice that kids have something to believe in but, when they are around 10 classmates make fun of them for believing in Santa.
I think I would tell her that Santa is something that parents have been told of for many generations and that carrying on the tale was a tradition. Tell her that parents used Santa in part because they couldn't always buy what the children wanted. Also that's why Santa said they had to be good all year. That was one way to get the parents off the hook. She will understand better than you think.
When my son was 5 he ask if Santa was real then why couldn't his friends get new clothes and a bike and things like he did. He said he didn't believe in Santa because Santa would give equal to all not just some. So I told him there was not a Santa but, Santa was used because of a tradition.
Don't tell her! Just think about it. You'd be breaking a little innocent girls heart! But if you must tell her.... Actually don't even TELL her, show her. Try to catch your parents in the act. Oh and you know that it's Summer, right?