He did throw her through the second floor wall of a building. Kimiko just walked it off. He was probably pulling his punches anyway.
He didn't own Homelander. He got the drop on him because Homelander is so arrogant he didn't regard the kid as a real threat. He dropped tons of concrete on Homelander and the next time we see him, Homelander doesn't even have a scratch. It's like me sucker-punching Jon Jones. I might hurt or even stun him if I'm lucky and catch him with his guard down. But as soon as he recovers, he will beat my head into a canoe without breaking a sweat.
Kimiko's brother had one power - telekinesis - to a high level. Stormfront has electrical generation powers, superhuman strength and durability. The kid was simply outmatched by a more powerful opponent. Also, it's implied that he gained his powers relatively recently. Stormfront, like the rest of the Vought Supes, would have got her powers as a baby injected with Compound V. She's had years of practice fighting and killing people with them.