upgrade Hayes MX4 to hydro, any gotchas?

Hi,

I am contemplating getting a small-frame 24" wheel Kona MTB for my 8 year old son. We live where dirt riding always implies hills, and he can't really pedal uphill on his single speed BMX bike.

So I found this used Kona, but I learned (from the BMX bike) that his hands weren't quite up to the task of using cable V-brakes on extreme downhills. So I installed a set of Magura hydraulic rim brakes, and he was able to do steep downhills with me.

Do those junky Hayes MX4 brakes use a 'standard' mount for the caliper? What about the rotors? Will I run into incompatibilities if he needs to switch to hydraulic disc brakes? Which brands have compatible mounts? This bike is probably 4 years old, if that helps.

Lifespeed