My husband says as his parents are providing the meal this year we have to go with a timetable that suits them
Well, yes, he's right I'm afraid.
I probably wouldn't choose to eat Christmas dinner at 8pm either! But if I'd accepted an invitation to someone's house and that was the time they were serving it, then I'd accept that and make it work. If you want to have Christmas your way then you need to host it.
I think at the ages of 11 and 14, your kids should be capable of a) not waking you up at the crack of dawn, and b) staying up late after an evening meal, so hopefully you can make this work one way or another. Maybe you could have a really nice Christmas brunch with your mum and the kids and have a chilled, casual day, maybe some nibbles in the afternoon, and then get dressed up later to go over to your in-laws? Might actually quite nice for the kids to feel like they're going to a grown-up dinner... or at least, that's how you could try to sell it to them!
I guess this way, at least you can have a relaxing day with the kids, maybe watch a film or play games, and the kids can enjoy their presents and so on uninterrupted before heading over to the grandparents. Maybe it's not ideal but I'm sure you can make it work.