The Marble Run Mystery Box makes a great kids toy. Unfortunately, the power cord is just two wires soldered to the motor. One wrong pull on the cord and you break a connection or damage to motor.

This quick print implements a simple power cord strain relief device. Run the power cord back through the channel between the motor mount and the tower. Remove the nut that holds the motor down. Place the strain relief print over the threads as shown, run the power cord through it, and slide the top on to the strain relief. You can use a touch of glue here on the strain relief top to make it permanent. Now screw the motor mount nut back down and your kids can pull on the power cord without damaging the motor.

A useful addition to this is a USB Extension Cable with ON/Off Switch! This one from Amazon works well and you get two of them for $10.

https://www.amazon.com/dp/B07X82661H?ref=ppx_yo2ov_dt_b_product_details&th=1