Some websites like LibriVox also offer public domain children short stories. LibriVox has audio versions of these stories, which is great if you prefer to listen rather than read.
You can check out Project Gutenberg. It has a large collection of public domain books, including many children short stories. It's free to access and download.
Your local library might also have some anthologies of public domain children short stories. They often have sections dedicated to classic children's literature which are likely to include such stories.