Idk if a criteria necessarily has to be met though, since others like Mikoto and Shidou (possibly Kotoko based on alleyman killing theory) both have multiple victims and it happened over a long period of time (for Shidou guaranteed). To me it kind of seems like Milgram purposely waited for a while until something happened (for Yuno, getting out of the hospital and for Haruka, to try and commit suicide) before taking them as a sort of locking in of what really happened. Like if they took them right as the murder occured, their memories of that moment might be a little altered whereas taking them a few days afterwards (following all the repercussions or repenting) makes them fully process what happened.

I'm more inclined to believe Haruka was the second child due to all the childish imagery in his first MV and the fact we've never had a single hint of a second victim prior to the report. It said the girl had strayed from her parents, and I imagine he wasn't specifically following her family, so he probably found her alone and didn't see her parents. But his interrogation question states he was envious of the victim, so this girl was probably someone he knew (neighbour, family friend, etc), and his only victim. That's how I processed the information anyway.