How to get rid of stinky feet?

My granddaughter is nine years old and her feet smell so bad that we set her shoes outside at night . I try to make sure she wears socks (clean ones) everyday . And I also make her wash her feet daily .Is there any thing I can do to rid her of this offending problem ? I have had her put sented lotion on her feet ( I can still smell the stink through it ) Is it a diet problem ? Any suggestions would be appreciated .