Raspberry PI controllable car (code for a 6-year old)
The implementation of forward is horrible. The only correct way of expressing it is to make all four motors go forward. As an analogy, how would you react if your keyboard manufacturer told you "oh sorry, we made a mistake, you have to type every second letter in uppercase to get a lowercase word"? That would be non-intuitive.

The API you provide must be as clean and simple as possible. You don't want your daughter to learn that "programmers make mistakes and are lazy about fixing them, so we have to live with that".

In which ways is your daughter supposed to change the current program? You should probably add some example code for waiting some time, to allow for composed commands like "forward, wait 1s, right, 1s, backward".

Your code seems to crash with an exception when an unexpected key is pressed. That's unacceptable for a robot, since it might harm people or pets when running uncontrolled (see Asimov's laws).

Using raw_input looks as if the user has to press the Enter key to confirm each action. That's too much of a burden. Controlling the car must be simple and immediate, to prevent accidents. How would you feel if your grown-up street car would ask you for confirmation for each step on the gas or brake?