I really don't know!
I own a dog, and I have young child. We are just getting to the stage of play dates. It has never occurred to me to mention the dog. Not because I assume people will love her as much as I do, or because I think she is human, or because I refuse to accept dogs could bite (despite what the sarky anti-dog posters on here seem to think). It is purely because she is just a part of our life and family, so it doesn't occur to me to mention her any more than I say "Oh, by the way, DD has a Dad who lives at home - is your child ok with men?" Having said that, I have sometimes mentioned it when I remember. It is just that she is a bit like background furniture for us and I just honestly don't remember to ask.
It clearly is an issue for some people - I will try and make sure I'm more explicit in future.
It is also worth noting though that once people have arrived, I don't automatically assume they will love her. Not telling people does not by any means imply that I think everyone is going to think she's wonderful. So, they arrive, she barks (well, sometimes she does. I actually like her to bark - I don't think it does any harm, in a fairly isolated house, to have people think there's a big scary dog inside). I make her sit, I hold on to her just in case (although she doesn't jump up) and then I open the door. Then I see what the reaction is. If the other person is holding out their hand towards the dog and grinning, I let her go. She wags her tail, runs over, sniffs their hand, receives pats and job done, and affection received, she buggers off and sits by the back window to guard the garden from squirrels. If there's any sign of discomfort whether from an adult or a child, I'll ask. Depending on the response I get, I will either take the dog away completely (if someone says they are scared), or I will hold on to the dog more firmly to let the person approach - more to let them feel that she's being held rather than because the dog is likely to get up and come to them once I've told her to sit. This usually happens with children, with parents encouraging them to come and give her a stroke. If they don't want to greet her though, that's fine and we can put the dog elsewhere. Actually though, once the greeting is done, the dog tends to either keep herself to herself or follow me round.
I would hope that people who had allergies would ask in advance as shutting the dog away won't help - there's going to be fluff and not much I can do about it so I'd see it as part of protecting their own health to just check.
If the breed is relevant, she's a rough collie. Looks exactly like Lassie, and is just as dopey although if your child falls into a well, don't rely on her being able to rescue it - she hates getting wet or dirty. The biggest fuss we have ever had from a child (and yes, we did shut the dog away but he didn't like her even being able to see him from behind a glass door - not much we could do about that) was because he wouldn't believe that she was a dog and remained convinced she was a baby lion. Now that, I might have mentioned owning....