The Children (2009)

Two families gather at an upscale English estate in late December. Elaine (Eva Birthistle) and her sister, Chloe (Rachel Shelley), along with their husbands and children, are set for a weekend of family bonding and winter fun. But when they arrive, one of the children becomes sick, and all of the children exhibit strange behavior. The adults don't realize the disturbing truth until it's too late: The children have contracted a disease that has turned them into brutal, psychotic killers.