A C100 form is the form used to apply to the courts for a contact or residency order for children.
Usually the courts expect mediation to be attempted first (although this is not necessary if there has been evidenced domestic violence), but mediation can be a good way to sort out any issues with contact.
As part of this application, the Ministry of Justice requires the applicant to have considered mediation. With certain exceptions, this includes having met with a mediator and discussed the possibility of resolving child issues through mediation.