Across languages, onsets with large sonority distances are preferred to those with smaller distances (e.g., "bw greater than bd greater than lb"; Greenberg 1978). Optimality Theory (Prince & Smolensky 2004) attributes such facts to grammatical restrictions that are universally active in all grammars. To test this hypothesis, here we examine whether children extend putatively universal sonority restrictions to onsets unattested in their language. Participants (M = 4;03) were presented with pairs of auditory words--either identical (e.g., "lbif [right arrow] lbif") or epenthetically related (e.g., lbif [right arrow] lebif)--and asked to judge their identity. Results showed that, like adults, children's ability to detect epenthetic distortions was monotonically related to sonority distance (bw greater than bd greater than lb), and their performance was inexplicable by several statistical and phonetic factors. These findings suggest that sonority restrictions are active in early childhood, and their scope is broad. (Contains 5 footnotes and 5 tables.)