Which is correct?
Few people knew the way, didn't they?
Few people knew the way, did they?
The question is difficult, because the example itself is unwieldy at best. As Peter pointed out, few and a few have opposite meanings in this context, so it would be unusual to use either where there is risk of confusion. Since few here means 'not many', and simple reference to negatives gives us Not many people knew the way, did they?, I would say few also takes did. Conversely, a few, meaning 'some', would take didn't. But whichever you mean, it is better to recast the sentence.
Few means almost none => implying negativity => Did they.
A few means, little but there is some => Didn't they.