What is the process in california family court to modify a visitation order?

You can make an ex parte request to modify the visitation order, but as a practical matter, if he hasn't seen the kids since July there likely isn’t any emergency just now, so you should simply file an OSC to modify the timeshare based on these current events. As for mediation, once the matter is filed you will be given a mediation date and you will be expected to have attended before the hearing. It is highly unlikely that the court will hear your request until you have attended mediation, so if you show up without having gone they will simply send you to mediation that morning, so you should plan on making that appointment.

California law requires that the court may not make any custody modification orders without first requiring the parties to attend mediation. The only other power the court has is its emergency powers. You can request emergency custody but you must demonstrate to the court that there is a significant risk of immediate harm to the children if custody is not changed on an emergency basis. Even iof you get an emergency order, the court will still send you to mediation and set the matter for another hearing to determine custody.

You really should talk to an attorney. If the non-custodial parent is out on bail and chooses now to come see his children, it would be helpful to have an order in place controlling his custody/visitation. Talk to a lawyer as soon as you can.