Get married
Upgrade your farmhouse to the second upgrade
Keep your spouse happy
Have at least 2 empty beds in the house
Sleep in your bed each night
Wait for your spouse to ask if you want to have a child
Say yes
Wait about 14 days for the baby to arrive
For a second child, keep the first child in the crib stage until the game offers another pregnancy/adoption event
For same-sex couples, wait for the adoption event instead of pregnancy