Translation:The child is clean.
The ㅅ is a base consonant, and when a base consonant comes at the end of a syllable block, it changes sound to a weakened or un-pronounced ㄷ, sounding like the t in the American pronounciation of "button". It is a sound change rule, all base consonants fall under this rule. Watch Learn Korean with GO! Billy Korean on youtube. He has many informative videos about Korean and sound change rules.