The world building and plot can be lost upon younger children because it gets complicated. There are different groups to keep track of — Red Paladins, raiders, nuns, Fey, and more. Even among the Fey, you have multiple groups, such as the Tusks, Snakes, and Sky Folks.
The plot also twists and turns. While this can keep adults on the edge of their seats, children may find it hard to follow. By the end, multiple people and groups go after Nimue, all of their motivations slightly different.