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
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.
Step 2: Makerbot Replicator 2
I used TechShops Makerbot Replicator 2 to print out the design I made in Inventor.
Step 3: From Underneath
This blurry shot from underneath the bike shows the new lock in place. Notice you have to reach up (in a very tight space) to unlock it.
Step 4: The End Result
As you can see here, he can no longer turn the knob to open the seat. He's not to happy about it but at least he can't get to the electronics.