Philippians 2:3-4, “Do nothing from rivalry or conceit, but in humility count others more significant than yourselves. Let each of you look not only to his own interests, but also to the interests of others.”
Load up the kids and take them to the nearest dollar store. Their mission, should they choose to accept, is to use their very own $$$ (or what you have given them) to purchase a gift for their sibling(s). The gift should be tailored to the recipients likes or interests. It could even be their favorite kind of candy bar. They must keep it a secret until Christmas morning when the rest of the presents are unwrapped. Have them include a handwritten note as to why they are thankful for that sibling.