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You do not state what the mediation session is for. Typically, the parties prepare a mediation "brief" and give it to the mediator several days before the mediation session. In that brief, the parties explain the relevant facts and point out the strength of their case and the weaknesses in the opponent. After reviewing briefs from both sides, the mediator will be in a position to help each side evaluate their claims, and hopefully help them reach an acceptable compromise. Be sure to consult your own attorney to protect your legal rights.
Mediation of custody disputes has its own rules and you should consult with a family lawyer or a free clinic in your area.
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It sounds like you are getting ready for a court ordered "mediation" which is now called Child Custody Recommending Counseling because the person facilitating your "mediation" may write a recommendation for the court based on your session with him/her if you and the other parent can't reach agreements in the session. A mediator is a neutral facilitator and would therefore never write a recommendation to a court. The best preparation I can recommend for this session is to bring pictures of your children and put them in the middle of the table and use them to remind yourselves that you will stay focused on your children and their needs, and that you will not put them in the middle. There are resources online for co-parenting, UpToParents.org is good and I have parenting tips on my website as well.