Make lists. Presents you will have to buy.Children are the priority. Food you have to get. Find out who you are going to spend Christmas Day with. Will you cook? If so plan a menu. Keep it simple unless you enjoy cooking. Make a cake if you wish to have home made. Otherwise, M and S or Waitrose are your friends! Tho there are others. Keep calm. It’s only a couple of days and the shops are open most of the time. Only spend what you can afford, if that is possible.
DH and I don’t tend to buy each other stuff for Christmas. I was thinking of doing that “1 thing they want, 1 thing they need, 1 thing they’ll wear and 1 thing they’ll read” for the kids rather than buying lots of tat, is that mean?!
Do you want leftovers? I like to send guests home with some so they can have a lazy day. But if you don't need leftovers 4 adults don't need a huge amount of food. The biggest issue with Christmas dinner is the size of the joints which means long careful cooking and limited space for sides.
Plan all the meals around Christmas. Start buying extra ingredients now, dry stuff to begin with, or things that can be frozen. Eat your way through your current freezer stash to make space.
Buy cards and stamps and do a few each night.
Sort out gifts to be sent abroad as posting dates are early.
Sort out children's gifts
Book appointments for hair, nails, tan etc if you need
Wash and iron bed linen, table cloths, napkins over the next few weeks
Check that you have enough oven/serving dishes etc
Lists lists lists are your friend
A few candles (leds with small dc) and some Christmassy music and you're half way there.
So that’s only 2 extras for dinner, the rest of it is as normal. Don’t panic, just buy a bigger turkey and a few more veg. If you’re not comfortable with the cookin find recipes, do a trial run and write out timings in advance so you’re just following a list on the day.
As for presents for the kids, whatever you would normally do really?
Work backwards a bit OP. What sort of Christmas Day do you want? Big breakfast and late dinner? Small breakfast, big lunch, leftovers for tea? Booze flowing or just wine with lunch? Sitting down to tv / film after dinner? Going for a walk? Visiting friends / other family? Games? After that, plan out the days your parents are staying. Once you know this, you can plan meals, then make a shopping list.
With presents, I don't think it's mean to do the want, need, wear, read thing but maybe do stockings or something that will keep them busy and include things that mum and dad would never get as it adds to the fun. Again, start with budget, get ideas, then research ways to get the ideas to fit the budget.
Lastly, but most importantly, divide the jobs. You shouldn't be doing everything if it's too much. Who is going to buy and wrap the presents? Who will be going food shopping? Who will cook dinner? And who will be watching dc while it's being cooked? Much better to be clear on this now than bicker or feel resentful on the day.