The ornamental design for a children's building block, as shown and described.
FIG. 1 is a top plan view, the bottom being plain and unadorned;
FIG. 2 is a front elevation view thereof, showing the design. The back
elevational view being identical to the front elevational view;
FIG. 3 is a right side elevational view thereof, showing the design. The
left side elevational view being identical to the right side elevational
view;
FIG. 4 is a sectional view showing the interior of two of the design; and,