I can't find any statistics that directly identify any increase in crimes committed by children and teenagers.
What I did find is that the number of children in Young Offenders Institutes is falling.
https://yjlc.uk/resources/legal-updates/youth-justice-board-annual-statistics-2024-2025
What I can say for certain is that we now have access to millions of hours of footage of things happening that we never did before. Just looking at your examples, if you believe gangs of teenagers didn't shoplift 10, 20, 30, 40 or more years ago, you are completely wrong. What didn't happen in the 80s and 90s or even the 00s was footage of this stuff didn't become part of the public conversation because we never saw it. This makes it seem like it's a new phenomenon.
As for child killers, there's examples in every generation of it happening, and it's thankfully still incredibly rare.
Every generation has said "ooh, these kids are awful nowadays".