It's not bad, despite the strange romanticization of children. Haruka herself is adorable and lovable. I'm just extremely wary of the direction the mangaka seems to be taking things. She keeps writing the male lead as a terrible bully a la "he pulls your pigtails because he likes you".
Wataru is intimidated by Haruka's athleticism and skill for baseball, and hates that she is his rival for the Ace spot. So he's constantly saying things like, "It's pointless for girls to play baseball (you can't go to Koshien)" and harrassing her.
In one scene he kisses her to get her to stop crying (because that's how it works in the movies asdfgh) and when she punches him and then wants to avoid him, she's painted as the unreasonable one. The onus is then on the girl to be mature and make up with him. And Wataru does semi-apologize, but not for the right reasons. It's never addressed that he shouldn't have kissed her without permission. Not a great message for the YIMs reading this.
He also smashes her fingers by slamming a door shut right before a game, but "he didn't mean to!" so he doesn't have to apologize or take responsibility.
Oh, and the mangaka attempts to paint Wataru sympathetically as he cries about Haruka being better at baseball than him, and wanting to get stronger so she thinks he's cool. Because a girl can't be better than a boy and not damage his fragile ego. She has to succumb to "biology" and get weaker, or he has to prove himself more capable in other ways for the Natural unBalance to remain, and for her to remain an attractive love interest.
Wataru's friend Kazuhiro handles his crush on Haruka much, much better. He's just a better character overall, imo. He's strangely mature, with a sad awareness of his family's financial difficulties and resolve to drop out after junior high to work to support his family. But his crush on Haruka is harmless, and respectful. He is actually very sweet in that he tries to remain aware of his friend's crush on Haruka, and not overstep/hurt anyone. He also notably isn't intimidated by Haruka's charm or skill.
And besides the main character, who is an elementary schooler, crushing on a high school student, there's also a classmate who is "mature" and demands her feelings for their teacher, a grown man, to be taken seriously. Just. No. I'm not even going to get into why these paedophilic depictions are wrong on so many levels.
I'll be back to edit if my opinion changes, but for now, I'm...concerned. Please let Wataru not be the endgame love interest for Haruka. She deserves so much better.
Edit: Wataru still sucks and needs to be taught better. Ken-nii is much better than I thought, and his handling of Haruka's crush and Wataru disrespect for her was pretty well done, considering he's still young too.