怪盗レッド 1 2代目怪盗、デビューする☆の巻

It's an okay children's book. A lot of it is about the main characters fighting/not getting along and I found that a bit tiring. It's also very exaggerated, even for children's book standards, e.g. the male main character is sooo smart, he did advanced maths in kindergarten and chats with scientists from the US and Europe in his spare time :D The last few chapters are about their first heist and also a bit tiring as it goes on for a long time, but for a children's book it's still one of the more enjoyable ones.

It uses very simple language though, so it can be a great first book.