Effort is definitely sub linear. And costs even more so.

Cooking for four kids clearly has more raw ingredients, but you also have more opportunity to buy them in bulk. The active cook time itself does not increase much either.

Hand me down clothing and books are all shared. The kids also help watch, teach, and entertain each other.

I feel sorry for children that grow up without siblings. But even more so for parents that that choose to not have any at all.