Aw, poor Kouichi. ' At least Kouji's a biter. Someone messes with his big bro, and those little chompers go into overdrive.
... Kouichi. A LITTLE scary? IF YOU ASK ME, THAT MALL WAS A FREAKING NIGHTMARE. I pray that in all my fangirl sadism, I will never turn out like them... Picking on poor defenseless Kouichi, hmph! ...But if it's Kouji we're talking about... well... how much would I get in terms of blackmail...?
*gasp* AYCH EE DOUBLE TOOTHPICKS? Kouji, mind your language!
Well, I must say that I sincerely thoroughly enjoyed that story very much! And good for Daisuke!

First thing is there's a few typos. The first is "stepmon" instead of stepmom - must have confused that with a digimon or something. The second is "knonw" instead of known. The third I remember I can see from the review box down the bottom - "However his twin at him fondly". I think you mean However his twin smiled at him fondly or something. Also, I got the impression that Koji/Kouji and Koichi/Kouichi were actually talking to each other in the first scene, but whenever you mention names it's always Kouji, like Kouichi never said anything there, which I find hard to believe. Also, you say they're eight years old? That can't be; if there in the same grade and in 5th (Jap) or 6th (Eng), not to mention their actual appearance, they couldn't possibly be eight years old. And in comparing their heights with Zoe/Izumi they're not short for their age either. Are these AU characters or something?
Next is the style. While the story itself is a nice one-shot of Daisuke/Davis showing the brothers around Odaiba, the narration felt very intrusive, like you were trying to be another character offering opinions so often you might as well have broken the fourth wall. There was also a little bit of second person where there should have been third person. A lot of these thoughts or comments should be done as one of the characters' thinking, not a narrated sentence; even if it's in brackets.
Davis' introduction is a bit strange mentioning ADHD (even I don't think Davis is like that) but Kouji and Kouichi seemed in character, though Davis was a bit strange due to his dialogue, but that might be because I'm more familiar with the dub version.
Something else that interrupts the flow of the story is the overuse of Japanese inserts. It's fine to use Japanese names, but especially with Rina you overuse senpai which would only ever be mentioned in dialogue; due to the style of narration she's almost always referred to as Rina-senpai even when it's not applicable to the character referring to her (namely, you, the narrator; you should just call her Rina, because I kinda doubt she's your teacher/master). Niisan also pops up too much. When I imagine the characters actually doing these things in an anime episode, they would never talk english one second then insert a japanese thing for something specific like brother. It's very jarring in that sense.
The dialogue itself was good and there was sufficient description to know what was going on. I would've liked to see more about how their family situation had progressed since the anime's events - providing this is set after it, correct? What particularly would interest me is how Kouji would respond to his father lying about his mother's death - unless that detail was changed in the dub. Ugh.

Awww, this story was so cute! The fluff!

That was hilariously adorable.

... I wish I had been there for the mall scene. I wish that very much.