With £50 I think I would do this:
Xmas day food: whole chicken, potatoes, frozen carrots, frozen peas, Yorkshire pud, gravy. £10.
Box of chocs and everyone can have as many as they like in place of pudding (would have been my dream as a child - I'm v chocolate orientated) £3
Extras if you get more £: sausages, bacon
Presents - I'd aim for 4 each probably along the lines of: book each (charity shop), item of clothing (charity shop/aldi (seriously their clothes are good). 12 yr old: hair brush set with hair bobbles or stationary set for 12 yr old, DVD (£land now do reconditioned DVDs and CDs and have fairly recent ones in or charity shop. 5 yr old: colouring book and pens, cars (charity shop/aldi).
12 yr old: £15
5 yr old : £15
Toddler £5 - rewrap older kids old toys if you have any that have been in storage, freecycle, homemade play dough, stickers from £land
Stockings - fully edible. Get sweets from aldi (cheaper than £land) and satsuma and fill stocking.
Not the healthiest I know but though a lot of the ideas above are amazing they will come out over £50.
If you view something like this as the basics (so you know you have all bases covered) any extra cash can add rather than be vital for pulling it off.