The Bike not a good example. A stove is a better one I think.
We have to touch the stove. My 15 month old hasen't touched anything scalding yet, but I am waiting for it to happen. It's not like I am going to watch him touch the stove and then ask him what he learned I will make attempts to save him from himself, but that seems like the wrong thing to do.
So if your 15 month is about to stick a fork into an electrical socket, same logic applies? How about if he is about to run in front of a bus? How about if he is about to drink down some ammonia? How about put his hand in a wood chipper? Sit back and let him learn from the pain?
Does the kid need to have those things happen so he learns not to get electrocuted or poisoned or crushed or chopped up? No. You can teach him about the dangers and, in most cases, you will successfully get him to avoid them. You can do the same thing with the stove. Like you said, you absolutely will not just let him scald himself. You'll try to stop him. Because preventing pain is the name of the game. And most likely he will learn without ever scalding himself. I never had to learn to not touch the stove by touching it. It would be cruel to sit there and watch your kid get burned to "teach him a lesson" and as much as you want to invoke the analogy to provide some mental justification to God allowing suffering on some very trivial example of pain (like a skinned knee or a minor stove burn), you KNOW that even that is something you won't do as a parent. Its not right. And when you scale it out to more significant examples like in my first paragraph, the illustration because as morally apalling as the WT's "God is like the teacher letting the student give a chance to explain to the class his insistence on the wrong way to solve the math problem"