In this paper, we study the use of goodness of pronunciation (GOP) on child speech. We first compare the distributions of GOP scores on several open datasets representing various dimensions of speech variability. We show that the GOP distribution over CMU Kids, corresponding to young age, has larger spread than those on datasets representing other dimensions, i.e., accent, dialect, spontaneity and environmental conditions. We hypothesize that the increased variability of pronunciation in young age may impair the use of traditional mispronunciation detection methods for children. To support this hypothesis, we perform simulated mispronunciation experiments both for children and adults using different variants of the GOP algorithm. We also compare the results to real-case mispronunciations for native children showing that GOP is less effective for child speech than for adult speech.