If your ex is refusing to attend mediation, you must attend the MIAM yourself and get your form signed by the mediator. You can then go ahead and apply to court for the child arrangements.
See the C100 form for more information on the MIAM and to check if you meet any of the limited exceptions for attending. If mediation takes place, hopefully you will reach an agreement, if it doesn’t, the mediator will sign the C100 and you can apply to court.
The application is made on form C100, and the cost is £215 unless the applicant is on a low income or benefits in which case they may be entitled to fee remission, see form EX160. The court will make a decision based on the best interests of the child.
C100 form: https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/946762/c100-eng.pdf
EX160 form: https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/952045/ex160-eng.pdf
E160A Notes: https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/958038/ex160a-eng.pdf