Hello there. I got married to the father of my two children last year. The youngest already had my partner's surname, but I still had to re-register her birth to make her "legitimate". The oldest had both our surnames, so we re-registered her too, and changed her surname at the same time.
You get the forms for this at your local registry office - you'll pick them up first so you can take them home to fill in, then they'll make an appointment for you so they can re-register the children in their hand-written ledgers (this is probably the same way you registered their birth in the first place).
I did have to get my partner to sign the forms, but he didn't have to be with me for the appointment with the registrar.
I think you probably will have to get in touch with your ex for this, but ring up your registry office and they'll let you know.
I loved having my girls as bridesmaids - good luck with your wedding.