Explanation:

- Option 1: "at home"

- This phrase is grammatically correct. It simply describes the location of the children.

- Option 2: "since two hours,"

- This segment contains the grammatical error. Instead of "since," you should use "for" when referring to a period of time. It should be "for two hours."

- Correct Answer

- Option 3: "As it has been raining heavily"

- This part of the sentence is grammatically correct and serves as a reason for the children staying at home.

- Option 4: "the children are"

- This part is grammatically correct and properly follows the clause provided for contextual reason.