I agree with you, I would feel exactly the same. Your daughter needs to get to know them first. I would meet a few times with her before leaving her. If they are offended by this then get your partner to explain how you both feel and it wouldn't be fair leaving your daughter with people she doesn't know. They probably don't realise its not that you don't trust them but don't want her to be upset. If they aren't willing to meet you first together then they don't see her, simple as that!