First off, marital struggles in a family of 8 children do not surprise me. Marriage alone is difficult and kids make it more difficult to spend the time with your spouse that you each need. Having said that, however, this situation is pretty typical of American evangelicalism as a whole in that something other than Christ and His cross is elevated to first priority. In the case of John and Kate it is their children. For many others of us it is work, spouse, activities, or ourselves. Whenever Christ is relegated to anything other that first priority, it is only a matter of time before our lives begin to unravel.
John and Kate's children cannot save their marriage. Their children cannot help Kate forgive her husband if indeed he has been unfaithful. Their children cannot help John love his wife when he doesn't feel like it. Only Christ can do that. But when children are elevated to Christ's proper place we choose to ignore the power of God to restore and heal us when we need it.
Everything and everyone in this world will let us down. Everything and everyone in this world is flawed and imperfect. Everything and everyone in this world lacks the ability to give us real life.
Only Christ is always faithful, is perfect, is all powerful, and is eager to shower His love, mercy, and grace on our broken, imperfect lives.