@Laura2592 How old does the baby look? If it's really an infant, not what Airbnb calls an infant, I wouldn't be overly concerned about safety issues. A baby that can't yet crawl isn't going to be a problem re safety.
If the baby looks old enough to be mobile, or it's hard for you to judge, I would contact the guest and ask them why they booked with a baby when your listing indicates it is unsuitable for that. Tell them you're concerned because there are safety issues for children you can't be held responsible for, which is why you don't accept them.
If she says the baby can only just sit up on its own, so it would never be able to get itself in a dangerous situation, then I'd just let it go and slam them for not mentioning the baby when they booked, in the review.
If the baby is mobile, I guess you'll have to decide whether to tell them to leave, or just point out the things you're concerned about and hope that she is attentive.
I just went to stay with my daughter and her one and a half year old this summer, who is already quite a fast runner, and I can tell you that my daughter doesn't take her eye off that kid. Not all parents are negligent or let themselves be distracted from making sure their child is safe.