An occasion of this sort is not the right thing to take a puppy to, however well-trained it is. Too many people will scare it and probably cause it to forget it is house-trained!
One little boy is known to be scared of dogs, and he is not going to become less so meeting a puppy at a large family gathering.
He might well lose his fear of dogs if he visits his auntie in her home with his parents and can get to know the dog in his own time, but if you force him to be with dogs you are running the risk of making a natural childhood fear into something he never gets over.
The puppy must take second place here; frankly, unless your DD is treating it as a surrogate child she should be able to work that out for herself. If she can't then let her and her puppy stay at home!
This may sound harsh, so I hasten to add that I love both puppies, dogs, kittens, cats and children, but the children should always come first.