Children, unfortunately, often don't understand the concept of waiting. Similar scenarios to this happen very frequently, but I'm going to give you a specific example that happened last night.
There was a mom and she had about five children, (which, OMG, five children) and one child was getting a lunchbox. She set it on the counter, but the mom was doing a return so I hadn't scanned any of the items yet.
As I was ringing up the return for the mom, the girl kept scooting the lunchbox closer and closer to me. Seriously, kid, I see that you have a lunchbox. I even acknowledged the lunchbox in my most kid-friendly voice, ("OOH, what a cool lunchbox!") but she kept pushing it closer.
She pushed it so far across the counter that she pushed it INTO MY HAND!
Excuse me, young child. Back up and don't touch me.
...No, seriously, I'm gonna need you to not hit me with a lunchbox.