Try to avoid custody disputes. Your child badly needs a sense of stability. Challenge custody only if the custodial parent is causing obvious harm or repeated distress to your child. False accusations of physical or sexual abuse cause great emotional anguish for the child. If possible, don't split siblings unless they are adolescents and state a clear preference for living in different settings.
Books can provide reassurance and support.
Your child can read about other children of divorce who feel sad and scared but yet ultimately emerge stronger. You feel your child is depressed. Your child has any physical symptoms, due to the divorce, that last for more than 6 months. Your child continues to believe that the parents will come back together again, even though over a year has passed since the divorce. You feel the other parent is harming your child.