Jo waited until she was sure that Maxine was gone before she turned back to Sam, whose face looked a little bewildered.
"I am so sorry," she said, before he could say anything. "And I know what you're going to say-a kid like that with powers like those could be really dangerous. But you know what? I'm going to help her. So I'd really appreciate it if you didn't tell Dean."
All of this was said very quickly, to avoid any comments from Sam. Her expression pleaded with him. She expected that the brothers didn't keep much from each other (and oh, the things she didn't know there) so she knew that what she was asking was big. All the same, she had to ask.
"Please, Sam. She's only five. She deserves a chance to figure out how to use them responsibly."
It wasn't that Jo was convinced that Dean would kill a five year old. It was just that she didn't want to put him in the position where he had to make that decision. And she sure as hell didn't want him making the wrong decision.