I personally wouldn't want brakes on a cart; at least, not on a basic EE. I think better would be to be sure you have good sturdy, comfortable-for-the-horse, harness, with a wide-enough breeching...then be sure to allow the horse to LEARN gradually how to 'deal with' the breeching as the brake...as in, gradually increasing the length and slope of any downhill slope you ask it to handle.
In my experience,ANY brakes will add weight, (perhaps considerably so for a mini) to the vehicle, for one thing. There is a maintenance factor on hydralic brakes(if they could even be put on a MINI-sized basic EE cart?)...as well as the added weight factor. Manual brakes I've personally seen on a homemade EE cart were VERY clunky and VERY heavy, also.I admit, I don't know what kind/configuration a cart builder would put on; have you inquired about the type, weight they would add, etc? I suppose if I had pretty much ONLY quite hilly places to drive, and I could have a very lightweight(but sturdy, of course)EE cart that would not be made too much heavier by a decent, well-working set of brakes(whether manual or hydralic) would I consider them on a cart for minis of the size you mention.JMHO, of course.
Margo