I mean: siblings.
Not to discount that it’s great when you play with your kids, but it’s different when your kids play together, and in particular when you can’t play that they have someone to interact with.
Yeah, the group dynamics of, say, a family with two children:
1. Kid A plays with Kid B
2. Kid A plays alone and Kid B plays alone
As compared to a three-child household: 1. Kids A, B and C play together
2. Kid A plays with Kid B, Kid C plays alone
3. Kid A plays with Kid C, Kid B plays alone
4. Kid B plays with Kid C, Kid A plays alone
5. Kid A, Kid B and Kid C all play alone
So, in a bigger family, many more communication patterns between children can arise, which, among other things, might develop the child's empathy etc.Adding mom and dad as "players" would give us many more possible scenarios here (Kid A playing with mom, Kid B alone or with dad, etc).
I'm a father of two with a low income, and I would like to have a third child for precisely the above described reason. As a sociologist in my country put it -- somewhat harsh words maybe, but probably largely true: a single child might get spoiled, two children may start competing too heavily, but three children and above is company.
Not only do they play but they also learn to care for each other. Parental effort doesn't increase uniformly with each child – if you're lucky and have done a good job parenting them, children create a society and help with raising each other.