DP and I went to the register office today to start marriage proceedings 
We already have two children together.
The registrar told us that we would have to re-register their births after our marriage.
Obviously I can/will ask for more details officially but until we return there does anyone know why this is and if we actually have to do it?
I have no problem with this but as I am not planning on changing my surname and the children already have DP's surname is this necessary?
I've tried to google for information but I'm no clearer.
Again, there is no problem with doing this. I'm just curious!