I'm going to bow out of this thread now I think as it's starting to go round in circles.
I'll say it again though - most responsible owners would be either putting their dog out of the way, or supervising the dog and children,
Contrary to what some people seem to think, most (obviously not all or there would never be any accidents) people who have both dogs and kids are well aware of any risks and are not looking to put their own children in any danger, never mind any visiting children. There are always going to be stories to the contrary. Doesn't make it the majority of owners who would behave that way. Just like there will always be stories of dangerous drivers - doesn't make all drivers dangerous, and it's definitely not a reason to stop your child going round to someone's house (or stop them being driven in a car, to continue the analogy). But please, while you are busy displaying links to dog bite victims, bear in mind that me taking a photo of my child (and every child who has been to my house) with a caption saying "Look, here are children who have not been mauled by the 'friendly family dog' because we were aware of the risks and took the safety of the kids (and dog) seriously will never make news. I may also take photos of myself, my sister and every friend who ever visited either of us as we always had dogs growing up.
I will now be more mindful of informing people we have a dog but there will almost certainly still be occasions where it slips my mind - because, as I said before, she is just part of our home and until recently it would have no more occured to me to tell people in advance than it would occur to me to tell people that a man lives at our house (dd's Dad!).
I would definitely not be bothered if someone asked though - why would you be, especially if someone is nervous or has allergies. In fact, I'd say if you do have allergies, then it is important you do ask. As someone else said, I wouldn't inform someone of every potential danger point in my house (bread in the bread bin for the lactose intolerant, milk in the fridge for the lactose intolerant, peanuts in the cupboard for those who can't tolerate nuts, squash for kids who can't cope with additives....) - if there are allergies I'd expect to be told as a parent in charge of someone else's child for a bit. If someone is nervous enough to enquire in advance then it's sure as hell not a problem for me if they ask.
If I forget to mention it and someone comes who is nervous, I'm not nasty enough to then leave them on their own with the dog so please stop talking as though dog owners are all completely insensitive. I will, by the same token, try and remember to inform people that I live with a hound.