Got tricked by a 6 yo and yelled at by her Dad – What Just Happened?!

I recently went to watch Chhava with my family at a nearby PVR. During the intermission, my brother and I went to get some water from the free dispenser when a little girl approached me. She pointed at a specific cup of water sitting on top of the dispenser and asked me to give it to her. I told her it was already used, and instinctively, my brother offered her a fresh cup. But nope. She refused. She insisted that I fill that exact used cup. I hesitated but eventually gave in and did as she asked. A few minutes later, her dad storms over, and starts calling us names for giving his daughter a used cup. I tried to explain what happened, but he just kept yelling about how we “lack civic sense” while his daughter stood beside him… giving me the smuggest, most evil grin ever. I was honestly stunned. How are kids this manipulative already?! Like, why did she do that? Where did I go wrong?