So it means 'not have cried like this', not (As I thought) 'not have cried so'. I was thinking of 'так' as an adverb, so that 'не' would apply to 'так заплакали' rather than just 'так'.

So "не так" in this statement is as if the speaker was turning and pointing at the baby and saying 'not like this'.

If that was me speaking, I'd have wanted to say "вы бы не так заплакали, но еще!" I've underlined the words that (for an English person) would be the ones we would emphasise vocally, although I think we English tend to use tone to emphasise things more than Russians?

How would one say 'You would not have cried so much'? (Which was how I translated the original sentence.)

Thanks for your help on this, I'll try not to wear you out with too many questions...