Train a child in the way he should go, and when he is old he will not turn from it (v6).
This is one of those verses we must handle well. It should not be read as a 'promise', rather as a 'rule of thumb'.
Just as a track develops when the same route is used to cross a field, (the familiar track guides our steps), in the same way, habits learned young flow into our adult life.
And just as we can then choose to walk a different path across the field (for good or bad) so too can children choose to change their own steps, they too have free will.
Liz and I need to keep working at showing our children the right way to go. Our children are still young and we can easily exert our influence upon them. However, that influence will change and I have to trust that they will stay on the right course.
In truth, this verse gives me great hope, primarily because I know Him who inspired the words and His commitment to my children.