Your expecting a big group of people on christmas day and there are lil ones, so I dont think your family will mind bringing a plate/dish. People often like to bring a 'special' dish to share so show off their clever recipe or presentation skills too in my experience , and those who cant be bothered, well, it doesnt really matter if anyone has a poor attitude does it? Most people will be happy to help.
Have fun preparing all the extras like a fabulous xmas tree and decent quality crackers etc - lol p its a long way off, but you`ve got me looking forward to it!