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Picture of Power Wheels Lock
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My 3 year old son has a power wheels 4-wheeler. The knob on the back of the seat unlocks it to give access to the battery pack.  Of course he likes to take it off & play with the wires.  The solution is to replace the knob with one he can't open.

Step 1: Autodesk Inventor

Picture of Autodesk Inventor
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extrd 2nd tab.jpg
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I made it at TechShop. http://www.techshop.ws.
I used Autodesk Inventor, which is avail to members, to design the replacement knob/lock. After taking careful measurements of the original knob & slot it fits into, I input them into Inventor.  The difference was that instead of drawing the knob on top, I extended the tab on the bottom.  That way you can only unlock it from the bottom.
 
you need to modify it next so that more than one wheel spins.