Of course, there needs to be some control/supervision but if they want to built the LEGO masterpiece before breakfast why shouldn't they? And if you have to help them then do just that. Why does it matter if the meals are at different times? Unless if you have one of those rigorous schedules for the day as you have to travel, visit, cook, etc then let kids be kids on Christmas Day.
I'm looking forward to being led by the kids and letting them open as quickly as they want to and then play with presents as they open them as at least I'll know they like the toys. How does it work in your family? When do the presents get opened in your house?