Several times.
It's an application to Family Court.
Once submitted, it cannot be withdrawn without permission from the court.
A parent can apply to the court for a Child Arrangement Order which sets out who the DCs live with and arrangements for contact with the other parent.
The first step is that checks will be made by CAFCASS - social workers who check national records for any relevant info regarding the DCs.
Then there will be an initial hearing - at which both 'sides' will present their case.
The judge, or panel of magistrates, will then decide what further info they need.
They may order CAFCASS to write further reports - usually a Wishes and Feelings or more in depth Section 7 report. The DCs will be spoken to for this unless they are very young.
The court might order both parents to attend a Separated Parenting programme.
Further hearings will follow to consider the reports and progress before an order is finally made.
It's adversarial, stressful, and time consuming.
And, if a parent breaches the order after it is made, the other parent has to take them back to court for it to be enforced - at their own expense.