You have an unknown quantity in your new Lurcher, I would think that the last thing you could do at this time is to leave your crawling baby with a dog you don't really know ;) I'm sorry if this sounds harsh, but from what you've written, it sounds like you were too far away to have prevented it, what if she'd done more than nipped him? and let's face it from what you've previously written she is big enough to do some serious damage if she wanted. For her sake and your son's/other children's, please don't leave them in the same room unsupervised, this means you have to be where they are, not on the other side of the room or 'in the kitchen' ;)
Perhaps you could use a dog/baby gate to allow her to socailise but not not be with the children if your not actually with them. and If you have to go upstairs then the dogs shouldn't be left with a young child unattended anyway.
Sorry if this isn't what you want to hear, but an ounce of prevention is worth more than a ton of regret ;)
Edited to say I've just reread and it was 'the dogs' in the living room with your son, what if one of the others had defended your son and a fight had broken out, right over him? he could have been badly hurt

I would recommend either taking the dogs with you into the kitchen or your child, never leave a young child alone with dogs, its asking for trouble ;)
Regards
Dill