'their knack' is a perfectly correct and usable phrase in written English.
Generally, 'knack' refers to a talent or special ability that one person has, so in the phrase 'their knack' it is referring to the person's ability or talent. For example, you could write, "Their knack for cooking made all their meals a huge success".
Some paralegals were called "story writers" for their knack for inventing detailed tales of persecution.
Without light to read by, advertisements lost their knack of making people feel unhappy or inferior.
The Cardinals also showed again their knack for coming alive when it matters most.
Mexico is business-friendly by comparison.Brazilians pride themselves on their knack for dodging silly rules (they call it jeitinho).
While the pair, Adam (Adam 12) Bravin and the former rapper Justin Warfield, are not particularly original, their knack for catchy melodies largely redeems them.
With their knack for tipping balls into the air, the Giants' receiving corps remains the world's greatest undiscovered beach volleyball team.
This goes some way to explaining their uncanny knack for finding their way back home from foraging forays.
The Lib Dems seem to have lost their old knack for snatching byelection seats; their share of the vote fell in both byelections this year.
Since then, their hitters' knack for putting the ball in play and taking their chances has seemed inspired.
However we know less about their ability to break down teams prepared to scrap for a point than their wonderful knack for letting opposing teams have the ball only to annihilate them on the counter.
By the time the matter was settled, they could have each purchased their own well-lit hunting lodges for what they paid to squabble over their knick knack duck collection.
