The ornamental design for a children's play gym, as shown and described.
FIG. 1 is a front view of a children's play gym;
FIG. 2 is a rear view of the children's play gym of FIG. 1;
FIG. 3 is a right-side view of the children's play gym of FIG. 1;
FIG. 4 is a left-side view of the children's play gym of FIG. 1;
FIG. 5 is a top view of the children's play gym of FIG. 1;
FIG. 6 is a bottom view of the children's play gym of FIG. 1; and,
FIG. 7 is a perspective view of the children's play gym of FIG. 1.
The figures include broken lines that are directed to environment and are for illustrative purposes only; the broken lines form no part of the claimed design.