Look, you're entitled to find other people's kinks squicky. Nothing you've brought up so far is my cup of tea, for the record—heck, I'm gray-ace. It took me years to be okay with any shipping in fanfic.
However, because preferences about sexual activities are very personal and very subjective, "don't like, don't read" actually applies here. You can't objectively judge the quality of subject matter you wouldn't enjoy even if it were written well.
... I'mma get on my soapbox for a minute. Please don't be alarmed.
Ahem. You also shouldn't pass judgement on a subculture without learning something about it first. Do you know enough about how this kink is practiced by real, consenting adults to tell if it's being written well? I haven't looked into it extensively (not my thing, after all), but I find the whole subject of human sexuality to be intellectually fascinating, so I have come across a thing or two. For instance, this comic (NSFW) about age play on the sex-positive comic archive Oh Joy, Sex Toy (still NSFW). This was written by an adult who practices the kink because it empowers them, not because they or their also-adult partner actually want to have sex with children. That's a refrain I've heard about lots of different kinks: people do it because they find it empowering, and they do it as consenting adults with other consenting adults who don't confuse fantasy with reality and, if they're being good partners, do thoroughly discuss and agree to everything beforehand.
With that in mind, I skimmed this fic. It actually seems pretty decent to me both writing and content-wise up until the bit about the chocolate laxative in the cookies, where consent breaks down. That might be grounds for sporking if it continues down that road, but I stopped there because, frankly, I don't want the mental images. Up to that point, though, the writing was mechanically decent, there was what seemed to me like a fairly reasonable explanation for why the character might have that kink, and the other character was engaging in it out of curiosity and friendship, without coercion. Mind you, I never got into One Piece, so I don't know the characters, but since most works of fiction aimed at younger people don't detail the characters' sex lives, there's rarely anything in the text that explicitly rules out having a secret kink, just like there's rarely anything to rule out a character having an orientation other than straight. Therefore, it's possible.
Anyway, "people who like age play must actually be pedophiles" is basically the same stereotype as "people who like BDSM must actually be violent psychopaths" or "people who play vampire RPGs must actually believe they ARE vampires and also wOrShIp ThE dEvIl *clutch pearls*." In all cases, for the vast majority, it doesn't hold water.
TL;DR, if you're going to keep reading kinkfic, please make fewer assumptions and do more research before you get up in arms about it. Or just stick to ones you actually like! {= )
~Neshomeh