The parent that does not have custody of the children is normally given access or visiting rights by the court. The view of the courts is that normally every child has the right to see and associate with both parents. Exceptions can be made in a situation where it is likely that the child will face physical or emotional harm. If you fear for the safety of your children you will have to tell your lawyer so that he or she can ask the court for special terms to be written into the custody order. Examples of terms that could be requested are No access is given.
Any visits are to be supervised by someone like a social worker.
Times and places for visits are specified. It is very important that the terms of the order be specific and clear. This is so that there is no confusion if the order is broken and it has to be enforced. You can immediately go back to the court for help. The court can order the police to enforce the order.