Not a “child-like” question in my opinion. Actually it’s quite a mature and intelligent question. You should be very happy to have such a thoughtful son, and I hope he always is a source of great Jewish happiness to you!
Why does God seem to allow evil, and let bad people do bad things to good people? It boils down to, "Why is there evil?"
I think the answer is that God doesn't "like" to do open miracles. The word "world" in Hebrew, "olam," means "hiding." That is, God made the world as a place to hide from us, so that we can get reward (in the next world) for finding Him.
To do this, God needs to allow evil to flourish, for a short time. If lightning struck every time someone did a sin, no one would deserve credit for being good, because there would be no other choice.
So, what's wrong with that? As I mentioned before, God made this world as a place to hide in. That way, someone who truly wants to be good will be good, and someone who doesn't, won't. Then, God will reward the good person for being good, not because he was forced into it, but because he chose it.
So, although God did great miracles, like splitting the sea, and the ten plagues, He only did them for that specific generation. No other generation ever saw so many wonderous miracles. This was sort of a one-time deal, in order to teach that God is really there, is the Creator, and is in total control. From then on, though, God wanted people to use their free will to believe in God and follow the commandments, and not because they were "forced" into it.
