I love the idea. But, I suspect there would need to be some supervision if either of the kids are like, well, all other kids. It seems to me there are two ways to win. Dedicated focus and nose-pressing discipline or disruptive behavior that affects one's opponent.
I remember being challenged to staring contest by a 6 year old once. (Not mine. The daughter of one of the players I was coaching at the time.) It began. The girl talking trash about how awesome she was at staring contests and how she never loses.
Of course, she had never played against someone like myself with an INDOMITABLE WILL and enough petty pride to force a win for adults everywhere. We can't let the whippersnappers win. If they sense weakness...all is lost.
The contest begun with us each staring into each other's eyes. Her eyes get bigger, then mine. She moves closer and then I move a little closer--still staring. Other rounds of bigger eyes and closer looks ensue. After a bit, she realizes I am not like the others and I can see doubt on her face over her claim to never lose. Eyes bigger, move closer--until we are near nose to nose with wide staring and now watering eyes.
As I know she is about to surrender, I mentally prepare for my victory dance. (Mine resembles PeeWee Herman's dance to the "Tequila" song in his movie.) When suddenly, she licked my nose. Startled, I looked away; losing the match.
Wasn't like we had any rules against it. Even if we did, I bet she'd have come up with some "I'm not touching you"-type strategy that would have the same result.
In the picture of the game above, I'm not sure if a tickle or a quick tug of the hair would win--I just know it is a likely outcome if tried with kids I know.