They can apply for access through the courts who will decide what is in the best interests of the child. This is usually done in conjunction with a safeguarder's report.
It's best if you can reach an amicable agreement with the relevant parent / guardian.
thank you uno.who . have tried to reach amicable agreement with parent, but, sadly couldn't. could you advise me what a safequarder's report is please. thank you again Pat