The way that I see it is that they’re putting emphasis on it because he’s a young person that feels like he has to act so old and mature all the time. But Sougo wants him to act more his age (which we know is someone in college based on Sougo saying he was thinking he could enroll to be around more people his age). Also I interpreted the last part as basically Sougo healing Haruki’s inner child. Like Sougo loves Haruki so much that it’s healed the parts of himself that he closes off, such as his “childish” needs and desires. Essentially being that Haruki sees anything outside of him being mature as “childish” and Sougo just wants to indulge his “childish” side because he wants him to be genuinely happy. Not because he actually sees him as a child but because Haruki is a young person who’s had to hold so much in and on his shoulders that he just wants him to enjoy himself. Wow sorry for the rant
Also it makes a big fuzz on the uke being a child so much that it's starting to feel sus, and then the uke's suddenly drawn literally as a kid on the last extra, no smexy scene tho so at least daz good, but still, why emphasize so much?