It is time the children ___ in bed

It's time the children ___ in bed

Are
Were
Have been
Had been
...

I think the answer is"were" or have been because they similar to each other Because there are differences between"are" and "were":
1. The verb "are" is the plural variation of the verb "be" and is used in sentences where the subject is plural or plural in form and the action or condition happens in the present.
2. The verb "were" is the plural variation of the verb "be" and is used in sentences where the subject is plural or plural in form and the action or condition is completed in the past.
We can also say:
The kids(children) are asleep(this means in bed)