I often have about 20 for Boxing Day, mixture of kids and adults from about 7 through to 80.
last year for the first time I bought some ready made chicken pasta bake things from Costco - I needed 3 at £7 each for the 20 of us, and served with garlic bread, salads. It was supposed to be their lasagne (I usually make it myself) but they'd sold out so this was the closest thing. My mother was not impressed when I told her what we were having - she doesn't do pasta unless it's spaghetti bolognaise or lasagne but it was actually really nice, had loads of big chunks of chicken in and plenty of sauce and was nice and tasty. She has even suggested we get it again for this year!
It was really easy, much easier than making your own, not least because I'd usually do it the day before - which isn't fun when that's Christmas day! I've tried doing lasagnes and freezing the mince in advance, then just assembling and baking on the day but because I needed to cook 3 big tins of it, it just took forever, much longer than they would normally take.
Tortillas went down well one year except with my mother who is very fussy as she thinks she eats everything but it's actually only everything that you could find in a British supermarket prior to 1985. Get a load of tortillas - you could get assorted ones if you felt like it, or get some plain soft ones and some tacos, buy sour cream, guacamole, grated cheese, salsa - if you get long enough dates on things and buy several smaller pots rather than just one big pot you might find it really flexible as you can open as much as you need and keep the rest in to eat later, depending on how many of you there are in the end. Then get some salad - either buy bagged or get the dc chopping - dishes of chopped lettuce, tomato, cucumber, peppers etc. Put them all on the centre of the table and people assemble their own as they like them.
For the meat bit I'd do left over turkey if there were any that liked it (hell, they could add stuffing, sprouts and roasties if they fancied it and it needed eating up, there are no laws that it has to be authentic!) and a chilli or tasty mince, and maybe refried beans. Lots of fun, very social, easy to prepare a lot in advance, very little prep needed (just the mince really - which you could have in the freezer).
Would also do some garlic bread and mini sausages because they always seem to disappear rapidly...
Pudding - I always end up doing a large tray or two of brownies (my sister's simple but effective recipe), ice cream, hot chocolate sauce (melt a large bar of bournville, a walnut sized lump of butter, a small carton of double cream and honey to taste - about 2-3 tbsp - together gently, stir gently until mixed, serve warm, is the world's niced and easiest hot chocolate sauce) and some sort of fruit salad - strawberries and blueberries are a nice mix, or melon, kiwi, banana and pear is another. See what is on offer at the supermarket and what's easy to prepare... Oh and cream to serve too. But that then covers all bases - people can have as much or as little as they want, if they want to be healthy they can have fruit, if not the whole lot mixes and matches really well. And if there are adults who can drink, adding a splash of a nice fruit liqueur into the fruit salad or having it with it too can add another dimension.
Happy boxing day - hope it all goes well with it and with your new year arrival!