I'm a father of 2. If not for my mother in law I would have gone insane by now. Way harder than 1. 1 was mainly hard because we had no idea what we were doing. With 1 it is pretty easy for a parent to take over while the other rests. Not so with 2 kids.

Dealing with a rowdy toddler and a puking newborn at the same time is draining in a way that is impossible to describe.

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.

Do you have two or more kids? I say it’s different. For people with more kids, a single child seems like a walk in the park. Because in the worst case at least one parent is "free". With two or more kids the default is being fully occupied or if one partner takes all of them for the other to have free time, it’s insanely draining and the other partner feels almost pressure to make the sacrifice worth it.

This of course being said about the situation without any parents around. The step from two to three is worse for car/transportation and hotel room but I think it’s not nearly as hard as from one to two.

What you also have to consider: You’re gonna have bad nights. With one kid only, it’s fine. You can recover. With two, you simple don’t have room for that. The bad nights will eat you up.