Onomatopoeias are frequently used in daily Japanese conversations and are a part of children’s early vocabularies. Previous studies have revealed that the phonetic structure and rhythm of onomatopoeia promotes word memory and production, and that sound symbolism functions as a cue to infer word meaning. This study examined whether the acoustic feature of speech is another factor that facilitates onomatopoeic word learning in children. The focus of the study was on voiced/unvoiced consonant contrasts related to the size of the referred object (e.g., dondon-large / tonton-small). First we analyzed mothers’ speech while reading a picture book that included onomatopoeic pairs contrasted with word-initial voiced/unvoiced consonants. Mothers read onomatopoeias that referred to small objects with higher fundamental frequency (f0) and lower amplitude than those of large objects. Then three-year-old children’s understanding of the onomatopoeic pairs was examined. The conditions were 1) original (acoustic features are almost identical between the onomatopoeias of small and large objects), 2) high 50 (the f0 of the onomatopoeias of small objects was 50 Hz higher than that of large objects), 3) high 100 (similar to the high 50 condition, but the difference was 100 Hz). The results indicated that the f0 is a possible cue to infer the meaning of onomatopoeias related to object size, and that the acoustic feature of speech would facilitate children’s learning of onomatopoeias.