What to replace Novatech cable disk brakes with?

My Raleigh Cayusa bike came with Novatech cable disk brakes - they seemed ok
at first, but recently the rear brake has given up any pretence at actually
breaking once underway. I want to get this fixed as I mainly ride with a
'tow' bike attached to my saddle post, which my 4 year old son rides.

Naturally I am concerned about being able to stop when I want to - although
the terrain we are riding over is not particularly extreme, we do have some
long descents with constant breaking required. With the extra weight of tow
bike and son, I am very conscious that only the front brakes are doing
anything to slow me down, and would like some guidance as to whether a set
of hydraulic brakes would be considered overkill (and I should just fix the
Novatech breaks), or should I consider something like a set of Formula B4,
or Hope M4.

Thanks

Paul