In order for a child to be issued a National Insurance Number (NINo) automatically, the child must be part of a live Child Benefit claim when they are 15 years, 9 months old. They will receive their NINo automatically just before their 16th birthday.
If a child has been part of a claim, but is not part of a claim when they are 15 years, 9 months old, HMRC can be contacted to request a NINo. This process takes approximately 15 working days.
If a child has never been part of a Child Benefit claim (or a claim for a childcare service administered by HMRC) HMRC will have no record of them. An application can be made to DWP to obtain a NINo.