This was one of the most underwhelming arcs in the manga, and the anime continued that trend.
First of all, I don’t feel like this kid added much of anything to the story. Did the arc tell us anything about the characters we didn’t already know? Not at all. Perhaps we could argue that Yukki has grown to trust Yuno more. But surely there is a better way to show that than by devoting two episodes to this little brat.
The manga also goes through this segment where it has a series of short stories about the diaries. The first one with the evil cult was fine: it was an interesting story by itself, and Ninth, Fourth and the power rangers guy took part too. But this arc was just Yuno and Yukki vs. a little kid that no one cares about.
It doesn’t help that the little kid is annoying as hell. He isn’t believable at all. How would a little kid like him possibly arrange all this? Hooking up a chemical weapon to be sent through the mail? That’s not to imply that the rest of Mirai Nikki is believable, of course. It’s completely ridiculous. But most of the story can be pardoned because it’s exciting. Am I supposed to be worried about whether our heroes can defeat a little kid?
I don’t have much else to say. In terms of execution this episode was fine; but the premise left much to be desired.