S1 - Document not accepted by local authorities due to blank Personal identification numbers

Hello, I've applied and received S1 healthcare documents to insure my children who live in Greece with the local healthcare providers (EFKA). My children were born in Greece and hence do not have a NI number. I've submitted the S1 documents to the Greek authorities who refuse to accept them since the "Personal identification numbers" in the S1 documents are blank. What is the best course of action regarding this, e.g. can I ask them to use my NI number, or alternatively their local NI-equivalent numbers (AMKA) need to be used instead? Thanks and regards

Hi Ioannis Karydis,
Your children will be covered under your National Insurance records and the Greek authorities should use your details for the insurance. 
Thank you.