I had exactly the same issue. I applied for a child renewal on 20th February and was requested on 9th March for the birth certificate. It then took 2 weeks for the document to register as received in the application tracker. We’re now far behind were we expected in the process and looking like Easter holiday travel is not going to happen. If we’d been told in the beginning that a birth certificate was required we could have saved a lot of time and effort by providing it with the original application.