The ornamental design for a seat apparatus for a child's ride-on vehicle, as shown and described.
FIG. 1 is a top, front, and right side perspective view of a seat apparatus for a child's ride-on vehicle showing my new design;
FIG. 2 is a top plan view thereof;
FIG. 3 is a bottom plan view thereof;
FIG. 4 is a right side elevational view thereof;
FIG. 5 is a left side elevational view thereof;
FIG. 6 is a front elevational view thereof;
FIG. 7 is a rear elevational view thereof; and,
FIG. 8 is a reduced top, front, and right side perspective view thereof.
The broken-line disclosure is understood to represent a portion of the seat in which the claimed design is embodied, as well as environmental elements, both of which form no part of the claimed design.