My daughter's real surname is on her passport and bank account only. When I moved house when she was 3 I decided on a fresh start and school, doctors, dentist etc know her with her new surname. She's 9 now and knows the score with it. I assume that my Ex knows about it but he's never approached me about it. I did ask him for permission but he refused so I took the "known as" route and it works.
I am hoping that when she is 16 she will do it officially herself.