I think the 3 kids getting killed was based on all the other movies where teenagers get killed by zombies, serial killers , ax murderers, etc. Like the Halloween movies for example.
These are not those kids.
Zoe, Jack and Zach (Selena Gomez, Austin Butler, Luka Sabbat) are a trio of young adults who were driving through town, stopped at a motel and were killed there.
I am referring to Geronimo, Olivia and Stella (Jahi Winston, Taliyah Walker, Maya Delmot), a trio of teenagers who we first see at a juvenile detention center. They escape after the zombie outbreak, are shown in a few scenes sneaking through the town and at one point, one of them says something like “I know a place we can hide” and they disappear from the movie, to be neither seen or referenced again.
I would guess the 3 kids from the detention center probably had more scenes that were cut. The movie is only 103 minutes so it’s really short .
or maybe it was the Chuck Cunningham syndrome