Asking for some kind of family trip is a good one. I don't think there is any way to 'make' them stop this without causing problems that probably aren't worth it in the long run. Can you just donate some of the excess before the kids see it? Or have the kids be part of that process?

They may decide on their own to stop doing this. My mother kept doing this to me and it was very frustrating. She would spend a lot of money buying me something I couldn't return and didn't want. She would also be upset if I did not spend 'enough' money on her. On my birthday this year, she really cut back. I hope it's the same this Christmas!