Sounds Like a Problem I had.... GMing CHILDREN
In the summers I worked at a summer day camp as a counselor a few years back. One of my favorite things to do (and the kids') was to play D&D. I was the GM, and the kids learned to play on the orange 3rd Ed Boxed set (the next year we switched to OD&D, 'cause I'm old school like that).
Anyway, we had a problem of the youngest player in the game, age 6 (remember, the orange boxed set says it is for all ages!), for no reason at all said that he was going to attack his brother's character. I tried to reason with him. I said, "Are you SURE you want Rio to do that? You guys are on the same team!" He nods. "Cause if you do, he may get mad..." He shruggs. "And the other players might get mad..." Shruggs. "Your sure..." Nods. "Really Sure". Nods and laughs. "Really Really?" "Really Really." I sigh, "OooooooKay then, roll to hit"
Rio hit his brother's character with his battleaxe and rolled minimum damage. I was then SHOCKED by the decorum of the other players (age 8-12) who then only said, "Rio, we don't want you to adventure with us any more." They did not counter attack, or anything! Just told him to leave. This rejection must have hurt more than any sword wound (the kid probably just wanted to beat his big brother up for a change anyway). The six-year-olds face turned bright red, then tears began to well up, then the loudest crying I have ever heard was followed by the boy running out of the classroom, onto the playground, as fast as he could! Man, talk about a scene to have at a daycare type facility!
Anyway, I coaxed him back, and told the other players that he did that because he was charmed by the evil wizard they had fought last adventure. They knew it was a cop-out (kids are not dumb), but they accepted it anyway because they knew that the 6-year-old playing Rio the barbarian did not really mean them harm-- he just wanted to see if his character could beat his brother's!
Anyway, I think these kids had the maturity to come up with the answer your players were lacking. Maturity-- getting over petty squabbles and getting back down to what really matters: killing monsters and stealing their treasure.