It'll be OK, won’t it?

'It was ... , wasn't it?'
(The 'other reason' being that the three elements in the date ~ 5, 12, 13 (in the order that British people would write the date; not that matters for present purposes) ~ are a 'Pythagorean triple'. You may remember that the simplest right-angled triangle with whole-numbered side lengths is the '3,4,5 triangle' (since 3 x 3 = 9, 4 x 4 = 16, and the sum of those two numbers is 25 which is 5 x 5); the next one that works so tidily is 5,12,13 (whose mathematical 'squares' are 25 again, + 144 = 169).)

Answer 4 is immediately impossible here because the affirmative 'feed' requires a negative tag.
Of the remaining Answers, No.2 is right because the structure is : 'It [verb]'d, didn't it?'.
Answer 1 has a present verb; Answer 3 offers the wrong auxiliary.

... As in, 'There mightn't be a little bit more of that delicious pudding, might there (by any chance) ?'.

A classic case :'They can't, ... can they?'
(As also in: 'They can't just take away our rights / my licence / people's votes ~ can they?')

Beware in Answer 2: although the verb cluster in the 'outbound' part of the Question contains 'have', the key verb is in fact 'must', which is therefore replaced with the all-purpose 'dummy' verb 'do' (or, in this past case, 'did') in the tag.
Answer 1 lacks the negative; Answer 4 has the sense of the past, but the wrong auxiliary and the wrong pronoun.
Answer 3 also probably reflects and reinforces the rather 'baggy' main verb 'do', rather than the original active auxiliary 'must'. English speakers may well refer to 'doing Pythagoras' or Shakespeare or whomever, rather than saying more specifically that they have studied geometry or read any particular plays.

'You weren't, ... were you?' is the nub of this example.

Despite the heavy sense of obligation in this sentence, the correct auxiliary is 'is' (or, within the tag, 'isn't') ~ rather than either of the options at Answer 3 or 4; while Answer 1 is plain ungrammatical.
We have set this rare example within quotation marks as it is an instance of false English: not grammatically, but in that what it says is a confusion of two separate expressions. It has been a 'household saying' within the present writer's family since ~ many years ago ~ a neighbour from Eastern Europe coined it by mistake. She was a professional lady (a pharmacist) with a son at the same local primary school as myself, where the boy had not been doing well for whatever reason, so the headmistress asked the mother along for a meeting. The gist of this meeting was then told to my mother using the wording given in the Question.
The conflation is between 'pulling oneself together' (obviously a metaphor) and 'pulling one's socks up' (also a metaphor, suggesting that someone should take a few moments to generally 'sort themself out' and smarten up their appearance, perhaps after some accident or failure). Just for once, we don't suggest you adopt the phrase as given! But it is a memorable example of well-intentioned 'broken English', and you may quietly be glad to learn from someone else's mistake. We can all learn that way too!

The ' 's ' in the Question must, from context, be short for 'has' ~ so only Answer 4 fits.
The all-purpose 'tag' in Answer 1 is not adequate here; Answers 2 and 3 have the wrong auxiliary, and Answer 2 still also the wrong pronoun.

'We have, ... haven't we?'
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.

'One isn't, ... is one?'
This sounds strangely formal in English, like a politician wringing his (or her) hands over a situation where ~ inevitably enough ~ there are some potential 'losers'. This is a perfectly acceptable piece of English, but hardly a turn of phrase that would often or readily sound appropriate in the mouth of a non-native speaker.