Purpose: This study aims to examine how language background and grade level influence the voiceless sibilant fricative production of two groups of children enrolled in a Mandarin-English bilingual school in Canada. One group of children was exposed to Mandarin at home as heritage language (HL), and the other group had little to no exposure to Mandarin until they started learning it as a second language (L2) at school. Method: Eighty-two children in Grades 1, 3, or 5 as well as 12 teachers engaged in picture-naming tasks in both English and Mandarin. Their speech production of voiceless sibilant fricatives was analyzed using both transcription and acoustic methods. The center of gravity (CoG) and F2 onset of the vowel following the fricatives were measured in the acoustic analysis. Results: Both groups of children exhibited high accuracy rates in producing English /s/ and /[voiceless palato-alveolar fricative]/ and were able to distinguish the two English fricatives based on the CoG regardless of their grade level. For Mandarin fricatives, children in the higher grades achieved higher accuracy rates. Mandarin HL speakers had a higher accuracy rate than Mandarin L2 speakers, with their productions more closely resembling teachers' productions. The CoG distinguished all three Mandarin fricatives regardless of grade level and language background. However, significant differences in F2 onset were only observed in the HL group. Conclusions: The finding that unshared fricatives were more difficult than shared ones, along with the significant impact of language background--but not grade level--on the accuracy of fricatives unique to Mandarin, suggests that students, even in higher grades, can benefit from more explicit teaching. The developmental patterns and error trends identified in this study can serve as a reference for speech assessment and aid in planning treatment or instructional sessions.