The ornamental design for a child's garment, as shown and described.
FIG. 1 is a front perspective view of the child's garment shown as worn by a child;
FIG. 2 is a front elevation view of thereof;
FIG. 3 is a rear elevation view thereof;
FIG. 4 is a right elevation view thereof;
FIG. 5 is a left elevation view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view.
The broken lines showing a child depict environmental matter and form no part of the claimed design. The remaining broken lines represent stitching, and are part of the claimed design.