I have divorce with my ex-wife on 2018 March. We lived separately since 2016 November.We didn't have

I have divorce with my ex-wife on 2018 March. We lived separately since 2016 November.We didn't have any child in our widlock,It's clearly mentioned in Divorce decree.On 2018 August My ex wife has pre mature delivery of her baby girl which belongs to her partner.Unfortunately during first 2 days of delivery her partner was unable to present due to some urgent work.Thats why my ex wifes family family requested me for help.In humanitarian ground I admit her during that time.I filled up admission form and other papers as guardian/husband.Due to this mistake in baby's birth certificate my name came as father.Later on my ex-wife ,her partner(presently husband) along with me written joint application to Hospital authority to correction this mistake.But they refused to do so without Court order.Hospital authority told us to give three of us joint Declaration to Court on this matter and get order .Then only they change it from their end.Kindly advice what type of Case we need to file to do .