There is evidence to suggest that finiteness marking on verbs and subject-auxiliary inversion are related phenomena in English. In contrast, in Spanish there is evidence consistent with finiteness marking on verbs and the apparently similar phenomenon of subject-verb inversion being unrelated. In both cases, most of the evidence adduced comes from adult acceptability judgments and other adult psycholinguistic work. In the present article, we present evidence from child English and Spanish that supports the interrelatedness of finiteness and inversion in English, but not Spanish. Specifically, we show that child English speakers who are in the optional infinitive stage have variable judgments of finiteness that are predictive of their independently measured inversion judgments. In contrast, no such relationship appears to hold between the variance in finiteness and the variance in inversion judgments of child Spanish speakers of the same preschool age. We take this to be novel confirmation of the hypothesis that subject-auxiliary inversion in English takes finite tense as a necessary condition. In Spanish, in contrast, it appears that it is not necessary for a finite verb to move to the left periphery for subject-verb inversion.