I think it is fantastic to go away and break this cycle. Well done!
"I have suggested every year for about 6 years that we go to a restaurant but it’s always ‘too expensive’ so I’d be expected to pay for them as it’s my idea!"
Hmm. too expensive because they'd have to put their hand in their own pocket instead of yours for a change.
"It results in a truly miserable Christmas for me and DH!"
Hold on to that thought. Hold tight on to it. Because I suspect that when you tell them their freeloading days are over there will be squealing - much squealing! You have to have that single thought in the front of your mind - 'Christmas is miserable for me and DH and DC and you don't care as long as you get fed', and stand your ground.
As to how to tell them, I think WickedStepmomNOT's suggestion is the best:
"Send a family whatsapp or fb message (other social media are available) and say 'thanks for coming for Christmas, it was fun and exhausting at the same time with all the adults in the house and we've done it for a few years in row so next year we're doing something totally different - we've booked a small cottage to concentrate on our DC for Christmas while theyre still small. I know you'll be happy for us'."
This slides in the points that you've been hosting for years now, and that you don't get to enjoy your children on the day, and that they won't be small forever so it's now, now, now before they're bigger. I'd tell them ASAP, and maybe include that one of them should start a family Christmas Club, everyone chucks a fiver in each month and wow! there will be £60/head sitting waiting to pay for a nice restaurant come Christmas, how good is that!
One last point. Everyone's saying don't tell them where the cottage is or they'll want to join you, which is really good advice! Another thing to consider is that you need to be ready to laugh at them / tell them 'no, not a chance' if they suggest that they use your big house for Christmas dinner whilst you're away (I really am that cynical).
I hope you have a lovely Christmas 2023 just yourselves.