The correct sentence is: "The children had to collect rubbish, didn’t they?"

This is because "didn’t they" is the correct form of the tag question for the past tense. In English, when forming tag questions in the past tense, we use "did" in the main clause followed by the appropriate pronoun and the base form of the verb in the tag question. In this case, the main clause is in the past tense ("had to collect") so the tag question should also be in the past tense ("didn't they").

Using "had not they" would be grammatically incorrect as it should be "had they not" in formal English, but it is less commonly used in everyday spoken English.