Out of wedlock

Hey my name is Frans,in 2019 I went to home yo home affairs sign with my wife in August 2019 and after we decided to change my three kids surnames to mine someday.in the beginning of 2021 I received two birth certificates of last borns which are twins changed.and they told me they still processing the elder one.even today I haven't receive anything they still processing. Sad part my kid(8year girl) always asking gore why her surname is different from us.