The trolley dilemma gives the person at the lever no more information than the number of people that will die when he chooses either A or B.

Similarly, the bus driver most likely also didn't have information about his passengers or the child.

This is the only setting we can work with, when discussing the trolley dilemma, so bringing up information that is not included is rather pointless, unless you want to argue that the individual people in the bus are more likely to commit for example rape in their lifetimes than the child.

The invalidity of your argument becomes rather obvious when I just counter it with: "what if the child was actually the next adolf hitler?" As you see, the argument works both ways and adds weight equally to both sides, which makes it redundant.

Additionally, if you start to consider information that you don't have as part of a dilemma, every thinkable situation can become a dilemma. "If I walk out the door this could trigger a series of events that ends humanity, butterfly-effect-style, but if I stay inside something similar or worse might happen now what should I do?"

Back to my initial point, you are stuck with either having to assign a positive value to the lives of people, making 21 > 1 the right choice, or negative value, making 1 > 21 the right choice.