The ornamental design for a child's sofa, as shown and described.
FIG. 1 is a perspective view of the child's sofa showing our new design;
FIG. 2 is a front perspective view thereof;
FIG. 3 is a side elevation view thereof;
FIG. 4 is a top plan view thereof; and,
FIG. 5 is a rear elevation view thereof wherein the broken lines represent stitching.
The broken line showing of a child in FIG. 2 and portions of the sofa shown in broken lines in FIG. 5 are included for the purpose of illustrating environmental structure and forms no part of the claimed design.