A seat adapter removably mounts a child seat to a wagon having two opposing walls and an interposed connecting wall, and includes two latch components that mount to the wagon's opposing walls. In some embodiments, the seat adapter includes a cross-member extending between the latch components so that the cross-member provides structural integrity to the wagon if the connecting wall is displaced from its upright use position. In some embodiments, the seat adapter and the child seat can be mounted to the wagon for use with the connecting wall in its upright use position, and in some embodiments the seat adapter and the child seat can be mounted to the wagon for use whether the connecting wall is in its upright use position or its displaced position, with no mechanical interference between any part of the wagon and any part of the adapter or the child seat mounted to it.