Explanation:

- The given sentence: "It is time for the children to go to bed."

- Option 1: to being in bed – This option is grammatically incorrect. "To being" is not a correct verb form and disrupts the flow of the sentence.

- Option 2: should be in bed – While grammatically correct, this changes the meaning slightly to imply the children ought to already be in bed.

- Option 3: No substitution – The original sentence is correct as it is. "To go to bed" is a proper and common expression for indicating it's bedtime.

- Option 4: for going in bed – This is awkward and incorrect. The phrase "going in bed" is not standard usage.

Option 3: No substitution is indeed correct.