The answer is simple. Reach their child.
When you take a child by the hand, you take a parent by the heart.
I heard this quote by Bill Hybel's several years ago and have never forgotten it.
“Throughout the years, society offers the church just a certain number of entrance ramps into the ‘non-church’ world. At times, it's been sports. Then it was marriage enrichment. Today, I believe the single remaining common interest or entrance point for non-churched people into the life of the church is children. No matter how lost a guy is, he still usually loves his son. And no matter how off track a woman is, she still has a soft place in her heart for her kids. This means we have a wide-open door to almost every family in every community worldwide when we love and serve their kids.”