She said that santa is coming to her house tonight
Why would she think he's going to their house?
She is a big fan of Santa and she wrote to him to ask him to bring her some toys for Christmas.
It's so cute that she's that psyched about Santa.  Kids her age should be excited about Santa!  I hope you folks have a merry Christmas!
Santa isn't real, so I should just tell her the truth, and she'll understand later. It's not going to hurt her if I tell her he isn't real when he doesn't show up at the house. In fact, it might help her learn something.
Sometimes its okay to just let kids be kids. Letting them believe in fantasies and imaginary worlds and beings helps them enjoy themselves. Growing up and becoming an adult leaves less time and room for whimsical things and should be enjoyed as much as possible before its too late.