This study followed the development of Hong Kong Sign Language (HKSL) proficiency in 31 deaf bimodal bilingual children (kindergarten through Primary 6) over two years. The Hong Kong Sign Language Elicitation Tool (HKSL-ET) was used at three time points to elicit the production of agreement, classifier, modal, negation, and wh-question structures. These were scored for accuracy based on specific language features, including classifier handshapes, non-manuals, phonological parameters, sign order, and use of space. Deaf children of hearing parents (dH children) showed significant improvements in accuracy for all structures and features, with most progress occurring between the first two assessments. By the third assessment, dH children showed similar performance to the deaf children of deaf parents (dD children) on most structures and features, but they were still significantly less accurate in their production of classifier constructions and use of space. This, along with an observed negative impact of later ages of acquisition on HKSL accuracy, underscores the importance of early exposure to sign language. Examining the relationship between signed and spoken language proficiency revealed a significant positive correlation between HKSL proficiency and Cantonese verbal expressive ability, suggesting that these languages may have mutually facilitative effects in terms of language development. Our findings highlight the utility of bimodal bilingual education in fostering linguistic development in deaf children.