The ornamental design for a children's wagon, as shown and described.
FIG. 1 is a perspective view of the children's wagon with seats in a raised configuration;
FIG. 2 is a front elevational view of the children's wagon with the seats in the raised configuration;
FIG. 3 is a rear elevational view of the children's wagon with the seats in the raised configuration;
FIG. 4 is a side elevational view of the children's wagon with the seats in the raised configuration, the opposite side of which is an identical mirror image;
FIG. 5 is a top plan view of the children's wagon with the seats in the raised configuration;
FIG. 6 is a bottom plan view of the children's wagon;
FIG. 7 is a perspective view of the children's wagon with seats in a folded configuration;
FIG. 8 is a front elevational view of the children's wagon with the seats in the folded configuration;
FIG. 9 is a rear elevational view of the children's wagon with the seats in the folded configuration;
FIG. 10 is a side elevational view of the children's wagon with the seats in the folded configuration, the opposite side of which is an identical mirror image;
FIG. 11 is a top plan view of the children's wagon with the seats in the folded configuration; and,
FIG. 12 is a bottom plan view of the children's wagon.