欠の子 = Missing child

EDIT Something I find interesting is the repeated use of the number 9 on the vial and in the artist signature. The numbers 4 and 9 are considered unlucky in Japan.

4 is both よん [yon] and し [shi]. 死 [death] is pronounced as し [shi].

Same concept with the number 9 which is both きゅう [kyuu] and く [ku]. 苦 [suffering] is pronounced as く [ku].

欠の子 = Missing child