I've never really understood why people eat out at restaurants on Christmas Day. It seems to be a way to add unnecessary stress and cost to an already costly occasion – and let's face it, you can eat out any time of the year. If you want to see and taste the Christmas fare that a professional chef can turn out, lots of restaurants have Christmas in July functions.
Christmas for children requires flexibility so they can try out new toys. Christmas also means watching (or falling asleep in front of) bad television and old Christmas specials, visiting different sets of in-laws, swimming in the pool, frying on the beach or having a barbeque or picnic in a park.
So why do it?
Not enough room at your house? That's what parks and other public places are for.
Can't cook? It's about time you learned. Don't try to impress, just good simple food will counter all the chocolate and stuff that people get as presents. If you want Christmas cake and pudding, buy one to help a local charity.
Want to share the cost? Make it a bring a plate, or BYO drinks, or alternate hosting each year. All of these are simpler than trying to split a bill fairly, or justifying why the vegetarian should pay the same to go to a buffet lunch full of meat, or the one with a shellfish allergy should supplement everyone else pigging out on prawns.