You are probably well familiar by now with how English forms 'tag questions' ( ... aren't you? ... ), but here is your chance to practise examples in other timescales and tenses.
Don't forget that ~ unusually ~ English makes rather more of a brief grammatical fuss than many other languages do over this everyday structure: we need to switch between an affirmative sentence and negative tag (or vice-versa), carry the subject agreement right through the sentence ('He has, hasn't he?'), and even put in a 'stopgap auxiliary', usually in the form of 'do' ('It arrives automatically, doesn't it?') if there happens not to be one already available.
With this in mind, let's hope your own Answers will indeed 'be OK'!
Answer 1 is clearly hopelessly unrelated, even if we 'read into it' a compulsion always to revisit the same places ('must') and the dominant presence of the children ('they').
Answer 3 may almost be permissible in certain dialects; at least it catches a sense of the past ('didn't'), but 'us' as a subject is not normal standard English.
Answer 4 has the 'we' correctly, but the present tense is wrong ~ so it doesn't even match the start of the Question.