The Gods Lie by Kaori Ozaki

Natsuru Nanao, a 6th grader who lives alone with his mother, strikes up an unlikely friendship with the reserved and driven Rio Suzumura. Natsuru plays hookey from soccer camp that summer, and instead of telling the truth to his mother, he spends all his time with Rio and her kid brother at their rickety house, where a dark secret threatens to upend their fragile happiness.