Grammatical morphology has been shown to be problematic for children with specific language impairment (SLI) or developmental language disorder (DLD). Most research on this topic comes from widely spoken languages, such as English. Despite Welsh being the most extensively spoken indigenous in the UK after English, and Wales being the only official bilingual country in the UK, our knowledge about the morphosyntactic areas of Welsh that may pose problems for Welsh-speaking children with SLI is limited. Currently, Welsh-speaking speech and language therapists (SLTs) are heavily reliant on the use of informally translated English assessments. This can inadvertently result in a failure to take aspects of Welsh morphosyntax into account that are critical for the assessment and treatment of Welsh-speaking children. This is the first study to examine how Welsh-English bilingual children of early school age with typical development (bi-TD) and with SLI (bi-SLI) perform on production tasks targeting verbal and nominal morphology in Welsh. We targeted areas of Welsh morphosyntax that could potentially be vulnerable for Welsh-speaking children with or at risk of language impairment, such as tense marking and plural formation, and assessed their diagnostic potential. Twenty-eight Welsh-dominant bilingual children participated in the study: 10 bi-SLI and 18 bi-TD. They were administered three elicitation tasks targeting the production of verbal (compound and synthetic past tense) and nominal (plural) morphology in Welsh. The bi-SLI children performed worse than their bi-TD peers across all three tasks. They produced more uninflected verbs in the elicited-production task and were less likely to be prompted to produce the synthetic past, which is a concatenating, low-frequency form of the past tense. They also over-regularized less in the context of plural nouns, and when they did, they opted for high-frequency suffixes. By focusing on aspects of morphosyntactic development which are unique to Welsh, we have increased existing about how verbal and nominal morphology are acquired in Welsh-speaking bi-SLI and bi-TD children. The present results point towards productivity problems for Welsh-speaking bi-SLI children who are adversely influenced by low-frequency structures and fail to over-regularize in the context of verbal and nominal concatenating morphology. From a clinical perspective, targeting synthetic past-tense forms through a prompting task may be a promising assessment and intervention tool that future studies could explore further.