Changing Children’s surname

Can anyone give me any advice on how to change a child’s surname. I’ve been told I can just change it in 2 place like school and doctors and it’ll be recognised after 6 months but unsure of the truth in that. Seen on nidirect I can change name by deed poll but unsure if this will cause any issues with passports and such. Father is on the birth certificate so will require his permission. Thanks in advance