If it's going to be her tag phrase every time she gets into trouble - for everything from running into a zombie to dropping a blob of catchup on her favorite blouse, you need to make it something that sounds natural and won't become grating. Keep it short and simple. Uh-oh was already suggested and seems to fit the bill to me. Or perhaps, oh great. Short, simple, and natural. Just my two cents. jon

 
~jon
My Blog - Mad Utopia Writing in a new era.