George Brown Class Clown, World's Worst Wedgie, Book 3 by Nancy Krulik

In his continuing, but mainly disastrous, efforts to be an upstanding, responsible ten-year-old, George Brown decides it's time to earn some money. (There's also an expensive remote-controlled toy that he's been coveting.) But no matter what he tries--whether it's working at his mother's craft shop, opening a lemonade stand, or setting up a backyard circus for neighborhood kids--the magic burps erupt at exactly the wrong times and wreak havoc on George's entrepreneurial plans.