Here is a Toy paddleboat for your kids to play with. Created using the Drafting/Modeling program Vectorworks which can output .stl files. I have not had the chance to confirm the .stl files are in the right format. There were some options in the export window and I don't know if I chose the right ones.
Print each file. Wrap a rubber band around the back end of the boat, making sure you insert the paddle. See the picture to help.
You could also look a the image and cut the same thing out of a block of wood.