How to protect an electronic construction kit (a toy) from damages?
If you really want to make this 'kid proof' (I have more experience with 'student proof', that might be a slightly weaker test) you should limit the voltage AND CURRENT delivered by the power source, and select each and every component to withstand that current and that (reverse!) voltage. 

100 mA @ 4.5V is probably enough. Use two LM317s (one for current limit, one for voltage limit) or even a vintage ua723 for the power.

That sensing diode might be a problem. Would the 1N5817 not do? Or maybe you can put a small resistor in series with that 1N5711 to limit the short-circuit current to somthing it can handle.

For the transistors I would take something more beefy, maybe BD139/140.