Help with son's 18 V car
The diagram you created is what is known as a field wiring diagram.  This type of diagram is only useful in showing how components are connected within a system.  Beyond a wiring reference, diagrams such as this are usually almost useless in demonstrating logic or as a troubleshooting tool.

The language that this is frequently converted to is a schematic ladder diagram.  This isn't to say that there aren't other methods of stepping through the logic of your system -such as flow charts - but ladder diagrams are usually the most spoken language to convert from a logical flow to real world wiring. 

Ladder diagrams allow you divide and conquer in drawing and stepping through the functions you are trying to accomplish.  Everything in Ladder Diagrams flow from Left to Right and steps down.

A quick google search of ladder diagrams resulted in this link which may be helpful in demonstrating what I am referring to: http://www.seattlerobotics.org/encoder/may97/ladder5.jpg

If I were you I would also consider Paralleling each group of 3 6V batteries to make a single 18V source rather than 2 sources.  This would allow some simplification from the start.







simulate this circuit &ndash; Schematic created using CircuitLab