Post-Nuptials and Custody Agreement

My question involves a marriage in the state of:MI
My husband and I would like to draw up a post-nuptial agreement on what we both own
We have 2 small children and we are in full agreement in what custody issue that we want for them which is joint legal custody. How can we get a legal agreement dealing with the children in a post-nuptial agreement? We have seperated and divorce is filed but thought we would check into this, so that both of us can see if we can work it out if we can;t work it out then the custody issue is already in place.
We both strongly agree about the custody of the children

You cannot contract away your respective rights to seek custody of the children consistent with their best interest, nor can you bind a court to award custody in a manner the court does not regard as being in the children's best interest.