This mod that will bring family life to Mars with lots of features. New game is not a must as far as I can tell.

Current features:
Baby: new age group for colonist up to 2 years old. Babies don't need residence and don't show up in game other then as family member in parent's infopanel. When reaching the Child age group they will take residence with the parents kicking one other colonist if needed.
Colonists form couples, new born children are assigned to them and families are considered when colonists migrate to different dome/residence.
New colonist stat: Affection that increases when the the family lives together and decreases when they are separated; it influences the morale and can grant temporary positive or negative traits when very high or low. Affection is shown in the control center and average per dome. Low affection on two colonists will trigger separation, separation can also be manually triggered.
Migration takes families into account and trys to some degree to keep them together, but it doesn't try very hard since otherwise the new stat would be pointless and always maxed, life on Mars is hard...families sometimes grow apart. Basically one family member can go away for some reason and the others will likely remain in place or has higher to follow them and very low chance to go to some third place.
Nursery: now a residence for only for orphan kids and also day care for children in order to allow both parents to work at the same time.