I have 2 children with the same partner and we are no longer together. We were NEVER married and we have been off and on again. The oldest of the 2 children shares BOTH of our surnames, but the youngest (my son) carries only his fathers name. This is a problem whenever I have to make doctors appointments or registration. Because they share the same parents I feel they should share the same last names. How do I go about adding MY last name to my son's so that they will BOTH not get confused by the system or institutions I wish to register them for?:(

You and the father need to go to a courthouse and file a petition for name change (I say go together because both parents must sign the forms in most cases, for consent of parents). It's not uncommon and shouldn't be extremely expensive if the father won't fight you about it.