There is too much to explain about mediation for child custody evaluations in California. There are actually coaches that specialize in preparing parents for mediation since it has so much weight. I would suggest calling an attorney to ask for advice.
I would say focus on the child's needs and don't attack the other parent. Be reasonable and willing to co parent are the best things you can do. Use common sense.
This is just my opinion and not a comprehensive answer. You assume the risk because this answer may not apply to your situation depending on the facts.
You want to be prepared for this. Don't try to wing it. The other attorney answer is correct--there are coaches for this sort of thing (usually therapists can help--your military benefits office may have some people and resources who could work on this with you). I wrote an article about how to prepare, and it's attached to this answer. Basically, be to the point, be positive, don't bash the other parent, and anticipate the things she'll say about you which are bad and be ready with good answers for what you think she might say. Role play with a friend. Seriously. These mediation and evaluation meetings are nerve-wracking and so you want to be sure of what you're going to say. Best wishes.
San Diego is a recommending county. This means that when you "mediate" at Family Court Services it is really an arbitration. Check their web site at http://www.sdcourt.ca.gov/portal/page?_pageid=55,1524469&_dad=portal&_schema=PORTAL
If you and your wife don't come to an agreement quickly the "mediator" will make a recommendation to the court, and the court will probably adopt it. If you believe you can come to an agreement fairly easily you will want to work with her to get a good agreement. Remember that anything you say can be used to create a recommendation. As the others have said, a cooperative attitude focused on working out what is best for the child will help.
It is also important to be clear about what you can and cannot do for visitation. 50/50 is difficult with a school-aged child unless you live close to each other. Where the child has to live with one person during school time the other parent has to have the bulk of school vacations to approach 50/50. That can be difficult to sustain. Avoid indecision or ambivalence, and avoid agreeing to something you can't actually do. If the focus on 50/50 is driven by a desire to avoid child support, think that through carefully.
Don't assume that she had the idea to start a child support case. She may have applied for public benefits and that triggered the case. The less you focus on the other person's motives the more successful you can be in coming to an agreement. Good luck!