Maybe kids could appear as our colonists get maried. So instead of paying for 1, they first pay for 2 (the wife/husband) then they pay for 3 or 4 people (their kids). That would deppend on how big is the house you gave them in first place. The colony would feel sooo alive this way... Maybe the kids could be playing around, runing and even exploring dangerous places, so you recieve a quest to rescue them. They could be imortal, and if your colonist (their dad/mom) dies, the entire family realocates (vanish) and you have to start again. This would make colonies more lucrative, and would add motivation to complete their quests, so they can "upgrade".