I think it is the biological clock keeps them awake.
My little one, 9:30 to 7:30, no nap during the day.
I put him to bed at 8:00, he just messes about and would not sleep. Begin to quite down at 9:30, and soon falls into sleep.
I think sleep is the same as food. If they are not hungry, they would not eat; not tired, they would not sleep. There is nothing you can do about it, part from driving yourself insane. Relax and let them be.