Babysitting

I love kids, don't get me wrong, and looking after them for a couple of hours is fine, especially when you're payed a lot, but sometimes it's a bit awkward, you don't know what to expect or how to act especially around the parents, which I always find really hard, because I feel judged. I feel like if I say anything stupid they won't ask me back. Like today the dad said welcome when he answered the door, and klutz like I am, I said welcome back?!? What's wrong with me? It's so stupid!