Neal at Literal-Minded parses "need to pee like a racehorse" and in the process mentions a way he likes to tease his kids:
Of course, if need to pee like a racehorse is ambiguous in this way, so is need to pee really bad. I have fun attaching the really bad down low instead of up high, producing dialogues like this:
Doug or Adam: I need to pee really bad!
Neal: OK, go pee really bad!
Now that I think about it, I suppose that’s not such a good thing to tell little boys.