We usually set a 1,000 word (4 typed pages), and the assignment is they are on restriction until it gets done. If they finish it in one day, great, if it takes them 2 weeks, that's their decision...but until its finish, they stay on restriction (no TV, no radio, no phone, no computer {except to write the essay}, no friends, and they stay home). Usually our girls finish it within 1 day....its in their best interest.

I worry that your children are learning that writing is a punishment, when they should in fact learn that writing is a joy. In my experience working in a school this is not effective in the long run. I would ground my children initially and then get them to sit down and discuss the rights and wrongs of what they have done at length. Perhaps making a brainstorm of where they went wrong and then another of how they should have behaved.