The Wee Free Men is a book that I would have loved to have read to me as a child. I never had that experience, but while I was reading I was thinking about how much fun it would be to read to a child.
It's endlessly imaginative and funny. Every character has real personality and the way the world works is fun to discover.
Tiffany Aching is the kind of protagonist that children should identify with. She's got a strong sense of identity and doesn't let the world keep her down. It's fun to read about a character who uses problem solving and has a strong moral compass to guide her way through the world.
The world can get a bit scary for really little kids (probably don't read this to anybody under the age of six), but for any adventurous minds out there it is worth the effort. There are all kind of spooky monsters and settings, and the way Terry Pratchett weaves fairy tales from our world into this world is a great bit of fun.
There are some jokes here that would go over the heads of a younger reader, but they are fun to snicker at as an adult. The writing isn't overly simplified, and it is satisfying and fun to read. My first Terry Pratchett read was a great introduction to his works.