Link makes an excellent point when she says " -The hard thing is you may not be able to change your parents' reactions and how they treat your children-"
SO... what about giving grandparents limitations on what they can spend on your kids?
Yeah, horrible thought, but lemme clarify....
If you tell them that they can only spend about 20 per CHILD on christmas, and thier birthdays... maybe that would curb this awful flaunting of favortisim. There arent many BIG wonderful kid toys for under 20 bucks so the playing field will be level when it comes to gift exchanges. it ishard to tell someone they cant spend X amount of money if they want.... but if they refuse to acknowledge how thier actions are not only hurting your kids, but HURTING you more ( having to spend 20 and forge a letter... lie.. cover.. etc) then they need limits and you MAY be able to start there.
This behavior is damaging for these kids. If they refuse to curb it , then they should be treated as kids as well. Immaturity doesnt stop at age 21.
( sigh ) im sorry you are going through this. Grandparents shouldnt be the cause of frustration. That belongs to Mother / Father in-laws.