In my opinion and experience, I feel that 15 month are a bit young to understand that Christmas means they get presents etc, my daughter was just 20 months old at her second Christmas and she didn’t really understand why she was getting presents etc but was still happy to get them. The Christmas just gone she was 3 and half and she knows that "Santa gives out presents for good girls" and was really excited about the whole thing.
For all the Christmas since having my children, I have either asked the grandparents what they were doing and either asks them to come to us for the day or if they didn’t want to come to us, when we could go and see them.
Also it does depend on the type of relationship the parents have with the grandparents as to how easy it could be to arrange etc, after all for the sake of the children they all should make an effort to either see the child or to make the arrangements in order for everyone to see the child.
Either way, I feel it is down to the parents of the child to make the effort in order for the grandparents to see their children especially at certain times like birthdays and Christmas.