This study investigated whether third person singular –s and past tense accuracy and error types can reveal distinct developmental patterns of agreement and tense acquisition in younger and older Welsh (L1) sequential bilingual (L2) English children with typical development (L2-TLD) and in younger children with language impairment (L2-SLI_Y). A group of older (L2-TLD_O; mean age = 93.72 months) and younger (L2-TLD_Y; mean age = 67 months) Welsh–English (TLD) bilingual children and a group of young (mean age = 63 months) children with SLI (L2-SLI_Y) age matched to the L2-TLD_Y group were administered the screening component of the Test of Early Grammatical Impairment. Results indicated that the three groups differed in their production of third person singular –s and regular past tense but not in accuracy on irregular past tense verbs, when vocabulary skills were considered. The L2-SLI_Y children produced similar error types to the L2-TLD_Y children, who differed from their L2-TLD_O peers in this respect. L2 children’s vocabulary size, nonverbal intelligence, and item-level factors, such as frequency and morphophonology, differentially contributed to their performance across the various morphemes. We discuss these results within current accounts of language development and impairment.