Sounds like you have some fun plans for the holidays! The Painted Hills can be really quite spectacular in the winter with the colorful landscape often dusted in snow. Depending on weather conditions, your ability to access the heart of the area might change, but the area is open all year, although some building and event spaces will be closued during major holidays.
Keep an eye out on weather as your travel plans near, and plan to bring plenty of warm clothing, boots and something hot to drink. The open spaces you find at the Painted Hills can result in quite chilly winds and conditions. Historically the daytime temperatures in late December are in the low forties dipping into the low twenties at night. The drive time, again depending on conditions, between Bend and the National Monument is just under two hours, so you could plan for a full-day trip out and back. Otherwise there are a couple of places for lodging in Mitchell, just outside the area, but you would want to call ahead and check to see if they have rooms or are operating in the winter as these are really the only nearby options unless you plan to head further east toward John Day. Check out some sightseeing and planning tips at Travel Oregon and the National Park Service websites for further details and have a fun visit!