I'm a huge believer in kids getting game time, but nothing Kelechi has done so far has indicated that he's ready to start games leading the line for City. The only time he played a big part in a game he was ineffective. He should however get more time from the bench as he is a potential matchwinner & defenders don't know him.
Compare what he's done with minimal time played compared to bony...
Then consider how much more mobile, energetic, and generally enthusiastic he is when he's on the pitch. He's also proven he's a damned good finisher too.
What else do you want?