My daughter, now 13, was born on December 27. We have always had her party on her birthday or the weekend of her birthday. She has a couple of close friends with bdays on, just before or just after Christmas. The friends usually have parties in the beginning of December.
I found that a lot of parents need a break by the 27th due to all the hustle and bustle of Christmas, new toys, family in town, etc and we always have a good turnout for our parties! I was shocked when she had about 100 friends from school turn out to her 13th bday bash this year. We have always had a heavy turnout on the 27th or within that day or so.
I do separate her Christmas gifts and bday gifts. Birthday is set up in another area of our house with bday decor and birthday paper.
It is a bummer sometimes with Christmas babies but you can work with it. I see your situation as being tough this year since you will more than have another little one. I don't know how you might deal with 2 bdays as far as 2 parties, 1/2 bdays, etc.
Christmas week is a biggie at our house due to Christmas, daughter's bday on the 27th and our anniversary on New Years Eve (20 yrs this year!!).
Best wishes with your little ones. They grow up too fast!!