Protecting solar panels from kids who may throw stones to get monkeys off roof
The real problem is the monkeys. The collateral damage due to flying rocks would best be dealt with by trapping and relocating the monkeys. Or, it might be helpful to use an electric (chicken wire) fence to keep the monkeys off the roof. Other deterrents might be motion activated sprinklers or ultrasonic alarms.

Obviously, there is not a perfect solution, but reducing the monkey population should cut down on the number of rocks being thrown at your house.

An alternative to chicken wire would be (bullet-proof) polycarbonate. Supposing that you have a glass solar panel, I would particularly recommend replacing glass with polycarbonate, especially if they do break the glass, because polycarbonate is much cheaper and more durable. Polycarbonate will become hazy after a few or several years, so it will need to be replaced occasionally for efficiency.

Pros and cons (polycarbonate vs glass):

Pros: 


Polycarbonate is significantly cheaper than glass. 
It's durable/bullet proof. 
It can be cut to shape.


Cons: 


If you replace the glass with polycarbonate you will get about a 9%
reduction in power. But you would definitely want to replace the glass, because if you use polycarbonate over glass, it will reduce the efficiency by about 20%. 
It becomes hazy after several years and needs to be replaced. 
It expands and contracts, so you need to attach it with screws and allow room (around the edge) for expansion.