Divorce has a variety of adverse effects on children. Generally, their reaction can be strongly influenced by the manner by which their parents handle the divorce.
Everything has changed for them. Their lives are disrupted. Their lives have become unstable. They fear what will happen to them without both parents in the home to care for them. Some think the break up is their fault. There is no more "family" time with both parents present: trips, vacations, parties, school functions, sports, etc. There isn't enough money to go around. They don't see enough of the non-custodial parent. The custodial parent probably needs to work more, worry more about family finances and has less time to spend with the kids. Home life is more stressful as a result. In some cases they become the victims of bullying at school.
They worry when their parents can't get along with each other and any discussions end in a fight. Some bitter, immature, self-centered parents play the children against one another or try to alienate the child against the other parent. That can destroy family relationships forever. In many cases children are uncomfortable with sharing their parents with new partners that may come and go. Visitations can be a problem when there's a new spouse or partner who the children do not know well or even like.
We know that some children of divorced parents have more emotional and behavioral problems and do less well in school than children who live with both their parents.