which is the most effective way for parents to reduce the negative impact of divorce on a child?

There are many ways to mitigate the negative impact of divorce on children, some work more than others, but certainly, speaking is the main thing, letting the children know that they are not guilty of that decision and letting them feel they are the most important  for both parents , keep the routine with the children also helps, this will keep them emotionally in the same place, and if there are no routines, start one, never argue, or speak negatively of the other party in front of the child, this is going to put them in an uncomfortable situation, keep the authority and spread all the love you can to them.