I figured out Santa Claus wasn't real when I was 7. I was 90 percent sure of it, but I still held out some hope that he could be real.
The following year in September my grandma died (I lived with my grandparents since I was born). After my grandma passed, obviously nothing was the same at our house, especially that next Christmas. My grandpa was extremely lonely and my sister and I were, of course, very sad because we had lost our mother figure.
My grandpa did his best and he put out our presents under the tree for Christmas morning as usual. About 3 hours after we had woke up, he came up to me and handed me a box (unwrapped) with some model plastic horses that I wanted from Santa. He said, "Sorry, I think Santa forgot to put these under the tree."
I laid down on the ground and started crying .. mainly because I missed my grandma, but also because I had lost all hope that Santa was real.
So, in my opinion, if your daughter believes, try to help her believe for as long as you can.
Growing up sucks sometimes.