If you want to keep both family names you won't marry however When you do a Marriage Contract in-game you will be able to take his family name or vice versa. It will be a shared Family name after a marriage.
Divorce will be a mechanic for two individuals wishing to end their marriage. However, the breaking of a contracts has consequences and if both parties of the marriage contract do not agree to separate then the individual who breaks the contract may incur consequences.
When you create a child you're given three options of what to do with the new NPC. The options are:
Close it off, only you can possess its body at a later date.
Open it up to the public, After the child reaches age 15 other players will be able to find your child in the Family Selection Screen.
Share your Child Code, If you share your child’s code with a friend, then after the child reaches age 15 they’ll be able to join your family as your child.
You may also find additional Family Information here: Family