Recent studies established that making concurrent judgments of learning (JOLs) can significantly alter (typically enhance) memory itself--a "reactivity" effect. The current study recruited 190 Chinese children (M[subscript age] = 8.68 years; 101 female) in 2020 and 2021 to explore the reactivity effect on children's learning, its developmental trajectory and associated metacognitive awareness. The results showed that making JOLs significantly enhanced retention for students in Grades 1, 3, and 5, with Cohen's "d"s ranging from 0.40 to 1.33. Grade 5 students exhibited a larger reactivity effect than Grade 1 and 3 students. Children's metacognitive appreciation of the effect was weak. Firsthand experience of the reactivity effect, induced by taking a memory test, enhanced their awareness and calibrated their judgment accuracy.