How do you politely tell someone you don’t want to have your kid around their pitbull

My girlfriend's sister has a pitbull, and we are expecting a kid. With that being said, I don’t want to bring my baby to their house. They have kids of their own who are older, about 6 and 12. My girlfriend isn’t as anti-pitbull as I am; she keeps saying he’s never done anything to their kids, so why would he do anything to ours? But I do not trust these dogs, and it’s starting to become a big issue. Her entire family believes in the whole it’s the owner, not the dog bullshit. Anyway, I just need advice on how to tell my girlfriend and her sister that the kid will not be coming over to their house.