Experimental studies show that children have greater difficulty with "wh"-extraction from object position than subject position, arguably an intervention effect (e.g., Relativized Minimality). In this study we provide additional evidence of a S/O asymmetry in A'-dependencies from a novel source--sluicing. The results of our first comprehension study show that English-speaking 3-6-year-olds obey the "identity condition" on sluicing--that is, they disallow interpretations in which the elided verb or arguments are distinct from their antecedent. Importantly, our results also show a subject > object asymmetry and thereby support syntactic theories of sluicing that posit a fully articulated (but unpronounced) TP at the ellipsis site from which the "wh"-phrase has been extracted, e.g., "Someone wrote this paper, but I don't know who <_ wrote this paper>," as opposed to certain semantic/pragmatic theories that posit no such structure. Our second comprehension study investigates the role of animacy. We find that children's comprehension of object sluices, but not subject sluices, improves significantly when there is a mismatch in animacy features. Our results are incompatible with models that are solely frequency based but rather provide evidence for structure-based intervention effects. We conclude that subject > object asymmetries can be found even in instances in which the intervener is not overt, such as sluicing, and that [animacy] may be a feature involved in the computation of intervention.