Crystal Keepers: Five Kingdoms, Book 3

Good vs. evil is the central struggle for the whole series, and extreme bravery is required. As in Book 2, the abilities of tweens/teens (and one small robot here) are underestimated by the enemy -- always a mistake. There's also ample warning about what can happen when you let your AI get too smart and how free communication is the enemy of any dictator.

There's a skirmish at the beginning outside modern Zeropolis where kids in animal form attack horses and people, and some guards die. After that, all the fighting is done in high-tech, nonlethal ways: nets that capture; foam and tar that harden over targets; some tranquilizer darts; and a drugging. When a monster rampages through the city, there's a mention that bodies are seen motionless on the sidewalk below. A bomb with no radiation detonates in an area outside where people live. Lots of talk of what might happen to Cole if he's caught spying -- he fears being tortured for information. A girl with half a prosthetic leg recalls the potential danger of pushing yourself too hard in battle suits that enhance physical abilities. Talk of a supercomputer that wiped out a whole city. Much talk of the loss of family in another world; when you cross over to the Outskirts, everyone you know forgets you ever existed.