If there is space where the party will be make a separate area for the tweens/teens. They will be more comfortable that way. Have food they will enjoy; pizza, hot dogs, french fries, chips/dips, etc.
Entertainment: See if you can find a company that will rent video games for the night. Either TV based hook-ups or big console games like in an arcade. If room allows ping-pong, pool table. Card games such as uno or board games are always fun. I worked at a summer camp this summer and the kids loved the air hockey table. There was always a line to play it. Make sure the games are free or have a supply of tokens for them to use.
Check in to hiring some adult supervision. They are not going to want baby sitters, but uniformed guards might make them feel important.
The church I grew up in had a rec room for the youth group. We had a pool table, foosball table, pingpong, video game center (with the original Atari, that's how old I am), suffleboard, and a pin ball machine. We also had a sitting area with couches.
Another idea is to purchase Christmas crafts and have them set up in stations for the kids to do. Girls will especially like this. Surprisingly, a lot of boys do to. Oriental Trading (www.orientaltrading.com) has great craft kits you can buy and they are very inexpensive. For my summer campers I got some plastic shoe box size boxes and set up each activity in its own box. I typed up and taped the directions on the top of each. Make sure you have plenty of tape, glue, scissors, markers, crayons, etc. for them to use. Kids today love to fold paper too. I couldn't keep plain paper out for their free use because of this. I had to divy it out sheet by sheet or they would use my supply up each day during their free time.
My campers loved goop; water, corn starch, and elmer's glue. We made up several batches of it for rainy day time. We kept them in gallon size baggies. It can be divided up for how many people want to play with it. HOWEVER, it is very messy. You won't want to do it if they will be in nice clothes. I had kids that would end up with it all over their shirts and would have to change. BUT, it would occupy them for hours.
I hope these ideas help. Have fun with it and Merry Christmas.