Have another kid within 2-3 years. Over the next 16 years you as parents will have hours to yourself every day while the kids keep each other company. Single children spend many more hours a week lonely and the parents have to be their sole family play mates at the expense of their own pursuits, relationship & energy. Adults will never be as fun or engaging a playmate as another kid. If you keep the gap within 2-4 ye…
Depending on the children, there can also be a lot of fighting going on, which can be tiring for the parents.
I'd say the jury is still out on the question of the optimal number of children (including zero).