I did not have much time to work on this yet here is what I added.
The key is to try and match everything else in tone, color, sharpness, noise, and especially lighting
A very wide soft shadow around the girls similar to the shadow around Santa.
I took the girls and made the shadows darker, added clarity, increased contrast, slight adjustment to vibrancy, increased sharpness and texture.
The selection I did was not too accurate (you can tell on the left girls hair) and also did not work on the let foot that is sticking out to the left side of the image. It has shadows that have no match with what should be in the image.