Spend time not money on them.
So get a nice hot choc mix, or the lump of choc on a spoon type, bag of marshmallows and can of whipped cream to make naice hot choc, and enjoy together snuggled up watching a movie, as the 3 of you.
Do some baking with them, for some treats that you either all love, or that you've never made before.
Ask them what they'd like, both to get (let them know the budget issues or a rough guide) and to do.
Are they still Stocking people? (As in, Santa leaving one)? If so, try to gather the bits from now - keep an eye on the Bargains thread on the Christmas board for ideas.
And yes, get them involved. Doing some jobs, maybe thinking about making presents for each other, you and wider family, or making cards for them. They are certainly old enough to help out a fair bit in things like prepping and cooking the meals, tidying up, keeping their rooms tidy, folding laundry (you could make it a race to see who does the neatest job in 10 minutes rather than the fastest as that could be messy results!).
Bring them Christmas "shopping", but more about going into town and enjoying the atmosphere with a hot choc and a bun in a coffee shop than actually doing lots of shopping. Enjoy the choirs singing in shopping centres/main streets.
Get out for brisk walks in the fresh air.
Or bring them to a movie instead of a panto to save a few £s.