How do sitter's handle when dog bares teeth during meet and greet - smile or warning?

Could be either. You need to meet the dog again without the baby present because I would never want to be left with a dog I could not approach. During M&Gs, I have decided several dogs would not be a good fit for me. I remember one who would not let me approach and kept running away. I wasn't going to take any chances of being unable to care for it properly.

Don't take a chance, especially with a large dog like that. They are usually quite friendly and harmless. But it wouldn't hurt to make sure the dog responds positively to you when alone. Try taking the dog out for a walk, away from its owner and the baby.