How do you go about adding a middle name after the birth has been registered?

[Fillyjonk]

We want to add another middle name for both our girls, for reasons too complex to go into.

How do we actually go about this?

DD2 is under a year so I assume we call up the registry office and make an appointment?

DD1 is 3, so I assume it is more complex? I do know people can call themselves whatever they want etc etc but I'd kind of like it to be a little more official, and pref on her passport etc.