This may sound odd, but I've had a mini bike where the plastic piece over the filter had the choke diagram wrong, so when you thought it was off it was really on and vice versa. I'm assuming you have already looked at that though since you've had the carb off, so I doubt it'd be that. Just thought I'd mention it though.
It could also be a loose spark plug wire. Make sure it's pushed all the way down. And if you think the actual crank case may be filled with gas, remove the spark plug altogether and pull the pullstart a few times. It should kick out any liquid in the crank case via the spark plug hole.
And depending on how old the neighbor kid is, are you sure he didn't try to adjust the governor/throttle screws? He may have fiddled with the throttle cable/housing, or possibly turned the plastic screw adjustment on the carb which would mess up the idle causing it to not want to stay running.