.......bring presents because is Chirstmas? Can not it be that as they live in Town and where invited to spend some days as guest, bring presents was a nice thing to do?

I'm not so sure that the conexion present-Christmas was very strong on that period in England. I can't remember now any other Austen's novel talking about presents in Christmas.

I don't really think that being with your kids those days was very important, as some others have said, children in this "up-level" families were rise by nannies and governesses. And as those days offer Mr.Gardiner some days off job he probably is glad to visit his old town and go there just for a week with all the children (nurses,maids...) would be a madness for Mrs.Bennet and maybe too expensive (maybe they would need another carriage)