There are very few apologetics books written for kids ages 12 and under. However, the ones that are available are quite good!
Cold-Case Christianity for Kids: Investigate Jesus with a Real Detective by J. Warner Wallace
Cold-Case Christianity for Kids is written for 8- to 12-year-olds and corresponds to the chapter sequence of the adult version of the book, so parents and kids can study together (I love that!). Even though 8 to 12 is the target age range, you can absolutely use this with 6- or 7-year-olds if you’re working through it with them and helping them understand the concepts. It teaches the skills kids need for evaluating the truth of Christianity by first using those same skills to work on solving a skateboard mystery. Each chapter reveals something new about an old skateboard that the characters are interested in. The kids learn a new detective skill, then apply it to investigating the truth of Christianity. It’s an easy, engaging read that’s packed with value. You can read my full review of it here.
Lee Strobel’s Case for… series for kids
These books correspond to the popular adult versions (one of which I recommended here), but are written specifically for 9-12 year olds. They are excellent! Your kids can read them on their own, or you can read them together and discuss. Here they are:
Melissa Cain Travis’ Young Defenders series
Travis has written three illustrated story books that explain key apologetics concepts through an engaging tale (to be read in a single sitting). She does an outstanding job of this very difficult task and I highly recommend these for kids ages 7 and up.
JD Camorlinga’s Picture Book Apologetics series
These wonderful picture books are targeted at kids younger than all the others on this page. If you have kids in the 5-8 age range, start here! They talk about concepts at the most basic level, and have sweet, simple pictures to capture a young child’s imagination.
Pig and the Accidental Oink! (Volume 1) – Introduces the cosmological argument
Chameleon’s Can of Worms (Volume 2) – Introduces the problem with moral relativism
Possums and the Empty Tomb (Volume 3) – Introduces evidence for the resurrection
In addition to the above story books, the authors have a kids’ book on logical fallacies (i.e., errors people make in reasoning, and how to think more critically). The content of this one would be more appropriate for kids 8+.
For older kids, ages 13+, there are many more resources available. I’m currently in the process of reviewing these to make recommendations, but in the meantime, I want to point you to this thorough list from Ratio Christi.