Balance

Why is it God would allow one of His children that once had served Him to fall away?

It's the same reason He would allow a man that murdered His children to come to Him for forgiveness.

God allows us choice: we can choose God, or we can choose Not God. That means we can choose good, which comes from God and is defined by everything that God is, or we can choose the other way--evil. God never curtails our ability to choose for ourselves, from the moment we begin our lives to the moment they end.
God has created an environment in which each of us can choose the path of our own lives. That means in everything in our lives, all things are in balance: good and evil, right and wrong. Evil is not balanced with good because it is equally strong; evil is balanced with good because God gives us equal opportunity to choose either.

May we make our choices mindfully.