The correct option is
B didn't go
The second clause is missing a verb and thus, the answer must be a complete verb. Also, 'was' indicates that the tense of the sentence is past tense and so, the answer must also be in the past. Further, the sentence indicates that it was raining and so it is reasonable to assume that the children were not planning to go outside. Thus, the answer must be negative.
Option A is incorrect. 'wasn't going' is in the past tense and is negative but, the auxiliary 'wasn't' can only be used with singular objects whereas 'children' is plural. Thus, it will make the sentence grammatically incorrect.
Option B is incorrect. 'hadn't went' is itself a grammatically incorrect phrase. According to the rules of usage, if the auxiliary is in the past tense, the main verb should not be in the past tense. Thus, it is incorrect.
Option C is correct. 'didn't go' is in the past tense and is also a negative phrase. Further, it has no grammatical flaws. Thus, it meets all the required conditions and is the correct answer.
Option D is incorrect. 'have went' is in the present perfect tense. Thus, it does not have the correct tense and so, it is not the correct answer.