You seek their agreement and petition the court. You will need an attorney to do it right, and quickly. In the mean time you should have a power of attorney drafted to allow you to meet the immediate or emergency needs of the child.
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I am sorry to hear about this difficult situation. There are many things that an attorney would want to know in order to help direct you to the proper course of action. Is your grandson with you now? Is child protective services involved? If the parents do not make appropriate arrangements such as guardianship/power of attorney, child protective services will get involved and that will make everyone's life difficult. Do you think the parents will agree to a power of attorney and/or guardianship? Is dad the legal father (i.e. signed an affidavit of parentage) Are the parents in Macomb jail? An attorney would be able to visit them in jail. It is best that something is done quickly because at this time there is no available adult that has the legal authority to act on your grandson's behalf (and that is when a court will step in). You have an advantage as a grandparent and the only person interested in the care and custody of the child.
I would immediately seek a durable power of attorney (DPOA) over your grandson, with a provision in the DPOA that you are the named guardian, if one is ever required. If your desire is then custody, regardless of when they get out, petition for guardianship and sit the DPOA naming you the guardian, plus their incarceration.
Legal disclaimer: This answer is not intended to create an attorney-client relationship. It is meant to give the questioner some general information regarding their question and the questioner should follow up by consulting with an attorney who practice in this area of law. John Gormley is an attorney at Gormely & Johnson Law Offices, PLC, which is located in Fowlerville, Michigan. He practices in the areas of domestic/divorce law and bankruptcy in Livingston, Shiawassee, Ingham, Genesse, Jackson and Washtenaw Counties. Gormley and Johnson Law Offices is a debt relief agency helping people file for bankruptcy. Mr. Gormley can be contacted at 517-223-3758.