The age of the dog would not necessarily worry me. A pup can be just as much hard work at this time of year.
I think anyone getting a dog at this time of year should really consider leaving it until well after Christmas.
Think about it, most family homes are total chaos in the weeks leading up to Christmas - the decorations go up, flashing lights, balloons, tinsel etc. all make wonderful toys for dogs (not!!). Then there is all the Christmas food, chocolate (poisonous), turkey (bones not a good idea) etc. Labs LOVE food and will not think twice about pinching food left lying around work surfaces etc.
I would really question any breeder willing to let a dog go to a new home at this time of year, especially a home with children. You will want your son's Christmas to be special and to be honest you will not have the time to devote to a new pet and be with your son as much as you will want to be.
Christmas morning: you will be getting up and wanting to let your son open his presents, the meal will be being prepared/cooking, the relatives will be coming round - who is going to walk the dog first thing?
If you are going out where are you going to leave the dog? In the living room with the tree and presents (wrecked when you get home) or in the kitchen with all the food around? Dogs when left alone can wreck a home at any time of year (my first dog ate two settees, a chair, a carpet, the wooden fireplace, shredded a beer can on my bed, pulled up a carpet and massacred a 5 foot umberella plant).
If the dog has been kennelled with her sister then she might also start howling at night wanting to be back in her kennel. How will you cope with that?
If you don't have an outside kennel where you can put the dog when you are out then I would really wait until your house is back to normal after the New Year.