He doesn't "let" those types of things happen. It's important to understand 'free will' and why God gave it to us.
As a parent, I want the best for my child and try to raise them to the best of my ability and teach them what they need to know to be successful in life. But no matter how great of a job I do, ultimately the choices my children make are their own. I imagine our Heavenly Father feels the same towards his children as well