The Spanish subjunctive mood (SUBJ) is said to be highly vulnerable in heritage language (HL) acquisition. However, there is little controlled research on HL-speaking children acquiring the various Spanish SUBJ contexts, so we do not have a clear picture of when, how, or why heritage speakers (HSs) develop in the SUBJ as they do. This study tests the development of the SUBJ in two of the earliest acquired contexts by monolingual children--SUBJ with volitional clauses and adverbial clauses with future reference. Through an oral sentence-completion task administered to 50 school-aged child HSs, this study observes whether language-internal factors (modality, variability) and speaker factors (age, exposure/use, or morphosyntactic proficiency) influence acquisition of the SUBJ in the examined contexts. Although SUBJ is categorically used in the first-generation input the child HSs receive at home, school-aged HSs exhibit elevated optionality; the majority show a pattern of use typical of very young monolingual children, and there is wide variance among the child HSs across all ages. Overall, they exhibit slightly more optionality within epistemic modality (adverbials) than deontic modality (volition). Crucially, exposure to and use of Spanish and, even more so, a standardized measure of Spanish morphosyntactic proficiency were strongly associated with SUBJ use in both contexts by the child HSs. We argue that the observed vulnerability in these early-acquired SUBJ contexts follows from an interaction between the child HSs' engagement with the HL environment (including their resulting command of the HL grammar) and linguistic factors common to all SUBJ contexts.