I would be the uncomfortable friend that Coppertop is referring to. So yeah, I'm an only child, don't have much interaction with kids, and my interactions with kids have been...interesting.
The thing is, I've thought quite a bit about the type of parent that I want to be and how I want to behave. So I have the theory down on how I want to interact with them, I just don't know how that works in reality. Definitely, a lack of confidence is something that the kids pick up on, much like dogs smell fear.
Do some people have an innate ability with kids? Possibly, but as with other things, I think that that innate ability is merely a starting point. Some people have more innate ability, some less, but we all need to work at improving ourselves.