I needed a toy wagon for a project my wife and I are creating, so I designed and printed one.
The image you see is the model printed at 160% of original size--just because she needed it larger. Print one "wagon," two "axles," four "wagon wheels," one "handlebar" and one "tongue rod." The axles should be printed upright on a raft. Everything else prints pretty easily without supports or rafts.
Everything should push together as a friction fit (except the axles--which will turn and the handlebar--which will move up and down).
Add paint to make the wagon more interesting.