The ornamental design for a childseatheatingpad, as shown and described.
FIG. 1 is a front view of a child seat heating pad showing my new design, a rear view being a mirror image thereof;
FIG. 2 is a left side view of the child seat heating pad;
FIG. 3 is a right side view of the child seat heating pad; and,
FIG. 4 is a top view of the child seat heating pad, a bottom view being a mirror image thereof.
The broken away symbols in the drawings indicate the appearance of any portions of the article between the break lines forms no part of the claimed design.