This is a short book, so the five main characters develop slowly. There is just enough drama with parents to strike the right note. Kevin's mother causes the biggest problems, since Kevin has to lie to her in order to train, but she eventually sees his point that he has recovere from cancer and doesn't need to be coddled. Izzy's family is a bit more problematic-- her father's repeated assertion that "Fitzgeralds run" isn't very helpful to Izzy, and the fact that her family chooses to run a 5K instead of watching her competition is concerning. Mackenzie interacts positively with her dads as well as her birth mother, but hopefully more information will be relayed in future volumes.
There are very few middle grade books involving actual ninjas, and not very many about extreme sports, unless you count some of Matt Christopher's twenty five year old BMX bike books. Pam Withers did some good outdoor adventure stories about ten years ago, but Junior Ninja Champions is a rather singular book that will be welcomed by readers who would rather be out rappelling down a cliff or riding skateboards down stair cases!