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 people knew the way, did they? This is a negative start, just a few up from nobody! A positive tag works well. A same-way tag (few/didn't) would not work - as NEGATIVE same-way tags are rare, and usually sound awkward.
A few people knew the way, didn't they?/ did they? Both work. As 'a few' is seen as positive, the negative tag is fine, and denotes a checking question or something closer to a statement. "A few people knew the way, didn't they!"
As positive-positive same-way tags are perfectly acceptable, the positive 'did they' tag works. Probably said in response to an earlier comment, or evidence that somebody knew the way! But it's a checking response. A few people knew the way, did they? Oh good. That's a relief.