In most cases, parents can co-parent effectively. If one parent is chemically dependent or suffers from a mental illness, much more work will fall to the the healthy parent. If the impaired parent does not address the problem, the children may be placed at risk and you may be placing yourself in danger.
The law assumes that both parents are capable of parenting the children. If one parent is not, then it becomes the responsibility of the healthy parent to make the case for why it is not safe for the child to be alone with or be cared for by the other parent. This is difficult for the healthy parent because much of the behavior that led to the decision to divorce may have occurred in the privacy of your home. There are no witnesses. Also, many clients keep hoping that the situation will improve and are in denial over the severity of the symptoms. They may not have the records to show the other parent is abusing substances or alcohol or is engaging in bizarre behavior.
When your spouse's behavior reaches a stage where you doubt that he or she can safely care for your children, it's best to start keeping a journal of your observations. Note the date, time, who was present and whether there were any triggering events. If the event occurred outside the home, note the address and police agency involved. Many clients come in telling me that their spouse had an OUI (operating under the influence charge), but they can't remember the year or the police agency involved. In our area, one of possibly three police agencies might have responded to an accident (state police, sheriff or local police). They each keep separate records. Depending on the location of the incident, one of at least three court systems might have been involved. It is extremely helpful if you do not have the records of the incident, that you can remember the police or court agency and an accurate date, because copies of those records can be obtained years later.
It's also important to take remedial measures in caring for the children. If your spouse cannot safely provide care, especially for little ones, don't leave the children in his or her unsupervised care. It's hard to make a case that your spouse is not a responsible parent, when they have been providing child care while you work or attend school.
When you feel that you no longer have a full partner in this marriage because of chemical dependency or mental health problems, get help. See a counselor who can help you sort through your feelings. You are grieving the loss of the relationship and the loss of your dream of a healthy, intact family. A part of you will be struggling to hold on to both. This can impair your ability to make healthy decisions to protect your children. A therapist can help you sort out whether your fears and observations are valid and what action you need to take to protect yourself and your children.