As neutral as possible as I am not person a or b but I am a friend of one.
Person a has 2 dc, divorced from xh (person b) and remarried. Through mutual arrangement and dc preference Xmas is spent at home.
Dc go to xh Boxing Day and do 2nd Xmas, gifts, Xmas dinner etc. Usually all is fine and the arrangement has worked well for many years. Arrangement is not the issue.
This year b has decided that any Xmas present he gives must stay at that house, this is inclusive of books, clothes, toys. A thinks that as presents were given to dc that they should be allowed to take them home if they wish.
WIBU