[Robbin]

Since these are small children, I would try for a bit longer before you ask them to be removed.

We used a token system (with poker chips). Every hour (or two) the kids would get a token that they could use to "buy" a special treat (small amount of candy like 5 M&M's or tootsie roll or Smarties, etc.). You can use a timer, or "pay out" on the hour. ALL the kids earned tokens, even the ones who didn't have a problem with aggression. They could earn a token if there was no aggression. Aggression resulted in time-out, loss of token, and LOTS of attention to the one who was hurt. At the end of the day, they could also cash in for extra privileges according to how many tokens they earned
(5/5 might get 15 min later bedtime extra tv time, or extra story, etc.) so they could still get that long-term reinforcement and not feel like one mess up ruins the whole day.