If Krishna is the Father of all living beings, why is there so much pain in this world?
There are many ways to look at this question. First it can be considered that the father sometimes has to punish the son/daughter in order to teach him a lesson. Often the son sees this as pain and suffering, but in reality it is instructional. If we accept that the material world is only a reflection of our real home in the spiritual world and that we are spiritual - only temporarily in the body - then we can understand that our entire existence here is a training for us to see that the material world is a place of suffering, it is temporary, and we should aspire to get out and reach the spiritual world.
Another point is that Krishna places us here with instructions how we should live a God-centered life, or even just a pious life, according to scriptures, while honoring each other, the earth, etc. However, if we choose to ignore the instructions then the consequences are not God's fault, but rather the reactions to our own behavior. God is just letting the consequences come.
In order to understand this, one must understand the nature of true love. If one is forced or drugged into "love," that love is not real. You may agree to love but you are agreeing because there is a gun at your head, or because you are not in control of your mind. Only one who loves voluntarily in full understanding of their emotion and sacrifice is in true love.
Krishna wants true love. He wants us to turn to Him because we want to love Him and sacrifice for Him. For Him to get true love He must leave us alone to make our decision. If we are looking for happiness and love in the material world we cannot have pure Love of God. If we turn to Him, however, He will reciprocate with us.
Otherwise, He creates a place where we can try material love and enjoyment. Although conditions often appear very painful here, in reality we cannot get hurt because we are spiritual beings, only temporarily here in the material world. Krishna leaves us here to make our choices. We have the instructions (in scripture) how to live here in the best way possible, but we must freely choose to follow. He won't get involved - except when we ask Him to. Then things become clear and we can see things as they are.