I like “Cassidy’s Crib,” but that’s basically just because I’m a sucker for alliteration. Depending on how old she is, she might not like that too much, since a crib might mean she’s a baby still. I also like “Cassidy’s Place” because it sounds like a cool, downtown hangout.
My 11-year-old daughter uses “M…'s Studio” for the banner on her blog. It was “M…'s Playground” back when she was 4, but that doesn’t quite work anymore. (It wasn’t a blog back then but just a personal page of photos for family. I don’t want anyone thinking I had a 4-year-old with a blog.)