Honestly, just use their accounts to start playing in a sincere effort to fix the problem manually. They'll quickly learn that nothing they can do to each other is worse than your mediation. I call that "The Solomon Approach."
Yeah see, then you get me as a parent My boys were battling it out, in game and in real. I make them get off the game but stay logged on, used the axe to dig till I was at the bottom and dulled the axe. Did this for both, then made them listen while those zombie things came and killed them.
This only had to happen once. They never fought again.
Its been like that forever. I've never known any other way to obtain milk buckets. Nor does the Change logs make any references to alternative ways. Perhaps you played on a server with plugins that added milk bucket as a drop.