Putting the children in the middle of a custody dispute is always wrong.
If the mother is doing this and inappropriately involves the police, then this will have serious consequences for the mother and will emotional harm the children. For that reason, you may need to consider filing to modify custody yourself. If the mother is acting to destroy the children's relationship with you that is one sign of her custodial unfitness. If you do file for custody modification you should consult with a Michigan lawyer about getting court services such as a guardian ad litem for the children or a custody evaluation.
If you feel that your children may not want to see you based on their own independent feelings then that is a separate and different issue. You may want to consider talking to them, talking to a school counselor, a therapist, etc. Participating in family therapy may be very beneficial.
I assume you have a Judgment of Divorce and that it contains specific provisions for custody and parenting time. It may also contain a section which prohibits either party from interfering with the parenting time rights of the other. Review your judgment or, better yet, have an attorney who specializes in family law review it with you. Your best course of action might be to file a motion to enforce the judgment or a show cause motion to hold the ex in contempt for violating the judgment. Good luck.
Sign up to receive a 3-part series of useful information and advice about child custody law.