This is from Good Luck Charlie.
Teddy, Gabe and PJ came to a restaurant to see their parents to tell them Charlie was lost. But suddenly they saw Charlie was in the restaurant. They were very excited. At this moment they did not know what to do. Should they still tell their parents what had happened to Charlie, or should one of them talk with them, distracting them while others went to fetch Charlie?
Teddy went up to mom and said,
"Hey mom. We just realized you must be missing Charlie, so we brought her. "
Mom said, "Teddy, you're not fooling me."
Teddy said, "I'm not?"
Mom said, "Nope. You spent a little too much time with Charlie, so you are ready to hand her over to me earlier."
I can understand "You're fooling me": you're trying to deceive me, you're telling a lie. But I can't understand the negative one: "You're not fooling me": you're not telling a lie, you're not trying to mislead me, you're telling the truth.
But mom later said "you spent a little too much time with Charlie, so you're ready to hand her over to me earlier."
By saying this, mom negated what Teddy had said earlier, "We brought her because we realized you must be missing Charlie". She actually meant "you're fooling me".
So what did mom mean by saying "you're not fooling me"?