This is why you never talk to the cops. If the mom hadn't said anything there would have been no way to prove the kid was on some juice cleanse, he grabbed the wrong thermos, or he just liked that specific taste combination and there would have been no case. But when the mom admitted she tampered with the drink with the intent of someone else drinking it, she confessed to a crime.
Never forget:
https://www.youtube.com/watch?v=RkN4duV4ia0
http://i.imgur.com/HOLHd4k.gif
http://i.imgur.com/six2WTN.gif