I TOTALLY agree with Dragonfly.
Those who don't "believe" don't receive.
My kids are 25 and 16. They have never said anything about there being no Santa.
Through the years, when they were younger, there were many kids who told them that Santa wasn't real and it was only the parents.
All I ever said was, "If you notice, the kids who say that are also the ones who don't get any Santa gifts".
My kids never gave me any guff about going to bed on Christmas Eve. I simply wasn't having any of that.
My son is 16 now and it's not like he has a 9pm bedtime or anything, but he doesn't get up or spy or anything like that.
Why would he shoot himself in the foot?
He knows there will be presents for him in the morning.
At his age, in his heart, he knows the deal, but he also knows that surprising him makes me happy.
He also knows that he has a single mom who works her butt off and could very easily sleep right through Christmas if given half a chance.
Cooperating about going to bed was just always part of it.
I always told my kids, "Santa won't come unless you are asleep. It's that simple".
They can't be bombing around the house and expect presents to appear. Even Santa isn't THAT magic.
You have a lot of years left with your littlest one. My daughter is almost 10 years older than my son and she was so helpful in playing right along about having to go to bed and go to sleep. Many years they slept together on Christmas Eve.
It might be hard to change things up if you've let your kids be in the habit of being up, but the first thing I would do is get strict about bedtime.
As the older ones get even older, they can be "Santa's Helpers" when it comes to getting the little one settled down and to bed.
That's how my parents did it, that's how I did it with my kids.
Do I believe in Santa?
Heck yeah.
Very best wishes to you. I hope you have a lovely Christmas.