UBI would just be enough to live on. So you wouldn't have enough money to raise children. That's just one possible policy. Obviously if you want people to not

UBI would just be enough to live on. So you wouldn't have enough money to raise children. That's just one possible policy. Obviously if you want people to not have children you'd choose that policy. If you want people to have children you'd choose a different policy, like paying UBI to children.