I claim the ornamental design for a bleacher baby seat, as shown and described.
FIG. 1 is a perspective view of a bleacher baby seat showing my new design;
FIG. 2 is a side elevational view thereof, the elevational view of the opposing side being a mirror image thereof;
FIG. 3 is a front elevational view thereof;
FIG. 4 is a rear elevational view thereof;
FIG. 5 is a top plan view thereof;
FIG. 6 is a bottom plan view thereof; and,
FIG. 7 is a reduced perspective view showing the bleacher baby seat in use; the broken lines depict environmental structure for illustrative purposes only and form no part of the claimed design.