"Children aren't safe around me, son. Not when my suit is off. No, wait, that came out wrong. Don't run! I didn't mean it like that!"
And Tony doesn't just pass by Harley during his desperate flight from certain death. He awkwardly opens up to the kid and lets him join in a fun investigation into the Mandarin's attacks. Tony Stark is a man who has Captain America's phone number, and when he's being pursued by terrorists he chooses instead to grab the nearest boy and look for clues? He's already proven in the movie that he vastly underestimated his enemies, so he probably shouldn't bet a strange kid's life on his certainty that no cyberterrorists can track him to rural Tennessee.
Even after the adventure is over, Stark is apparently convinced that the bad guys are completely wiped out and that no one would ever think of targeting Harley, because he invests exactly zero SHIELD resources into protecting the kid and essentially disappears from his life like an incredibly snarky imaginary friend. They might as well have filmed a post-credits sequence where the kid is tied to a table in a Hydra base getting interrogated by a snake-headed man, because that's almost certainly what happened after Tony left.
"Wait, how do I use this?"
Ever the learner of lessons, Tony continues his child-endangering ways in Captain America: Civil War. The feud between him and Steve Rogers stems from a conflict over what to do with the Winter Soldier. Captain America considers him a friend, while Iron Man considers him an assassin mind-controlled by Nazis. Both sides have valid points, and it ends up turning into a fight. So while Cap recruits a team of highly trained soldiers, Stark makes a beeline for a teenage Spider-Man and practically blackmails him into joining his side. Again, to fight a man he's very sure is an unstoppable killing machine.