Do whatever you feel comfortable with; parents will generally be more impressed if you involve their child in a conversation in some direct way, rather than just talking to the parents about the child as though the child isn't in the room.
Talking in a more animated, excited and varied way than the relative monotone that adults use to converse seems to be a better way to engage a very young child's interest and make them feel happy/feel like responding to you.
Varying the way you sound out your words can also be used as a differentiator as to whom you're talking, if you're of a mind to engage with a baby by looking at it and making faces (babies generally find faces fascinating) but want to carry on a conversation with an adult or mix communicating with adult and baby;
(monotone, looking at the child) Hey, so this is your new arrival huh?
(animated, still looking at the child) Hiya. Hiya. Now then. What you doing? Are you smiling?
(monotone) I bet your washing machine hasn't stopped, ours didn't for about 3 years...
(animated, still looking at the child) Who's a little dribbler? Hey? Your cheeks love that chocolate, don't they? Yes, they do...
It's quite a common pattern, that seems to make the child engaged in about the half of what you're saying that sounds interesting to them, while the adult can focus on the normal sounding mature content without having to process too many of the actual words coming out of your mouth before deciding who the intended audience is.. Parents are generally pretty tired, and personally I'd welcome the mental assist especially as children and babies tend to take the focus of the room attention and parents devote a lot of brain power to operating in "watch and protect" mode.
I know I said earlier, do whatever you're comfortable with (and that's important) but if you're trying to convey a particular impression to the parents, factor in what you know about how they talk to the child; As a parent, I'll generally assume that someone is comfortable around children/has children of their own if they talk to them in baby voice rather than trying to talk adult with them, and if your intention was to put me more at ease that you can handle being around my kids, varying the way you speak to them is a good start. Equally, you might have friends who think you're an idiot for talking to their kids like a pre school cartoon would, and think it's silly. Doing so might thus have the opposite effect on them