Oh god - I really feel you! My DC are exactly the same.
What we do is slow things right down - stocking is opened in our bedroom. Pause to play with toys and for dh to put on coffee / start breakfast.
Father Christmas toys are put in a sack so that they can dip into them when they are ready and aren't visually overloaded by all the presents (only a few). The presents from family are under the tree and mummy + daddy presents are put in another place. This sounds like loads, but it's only a few.
We let the DC pause to play with and enjoy each gift before moving to the next. We don't rush them through and opening presents can take two days if that's what they are feeling.
We pause for breakfast, sausage rolls at 11am, and a walk before or after Christmas dinner.
I'll put things in their advent calendar that hints to the main presents eg. Peppa Pig figures and the main gift will be a play set for them.
They get a Christmas Eve box which has a few bits they'd otherwise get on Christmas Day - toothbrush, bath stuff, new clothes, pjs etc
I think the key is letting them go at their own pace and not letting adults egg them on to churn through the gifts.