I observed a mother and her teenage daughter interacting the other day, and suddenly understood why kids can tell total strangers the secrets they won't tell their parents. It's because our kids love us.
When our kids love us, they don't want to hurt or disappoint or worry us, or have us think less of them. And they know that no matter how good a face we put on it, deep down we have a reaction. When a child is hurting, he or she can't afford to deal with our reactions on top of that.
That is why a parent can't be a child's therapist or complete confidante: because love gets in the way.
We will always know our children better than anyone else. But a child can count on a professional to be impartial. There's no risk of losing desperately-needed approval and affection. Which means that sometimes the reason kids don't confide in us has nothing to do with lack of trust, but with an abundance of love.