Adoption

Adoption is an option in both The Sims and The Sims 2.

The Sims
Sims get calls from adoption centers randomly asking if they want to adopt a baby.

The Sims 2
Sims can adopt a child over the phone if the call the Service and then choose "Adoption Center". They can adopt a baby, a toddler or a child. They could only do this however if the Social Worker had not taken one of their kids.