The first thing I would do is stop doing exercises that hurt the clients and investigate the cause of what you call ‘weak wrists’. Often, people have restricted range of motion and are uncomfortable in a position where the wrists are extended to 90 degrees as in plank.
As you noticed, forcing the wrists into that position does not work. If you are familiar with Thomas Myers ‘Anatomy Trains’ you may find the cause in imbalances in the arm lines. Working on those may improve range of motion and lessen the pain during some of those exercises, even I often see that people still cannot tolerate full body weight with ease. My first approach would be to acquaint the client with the MELT hand treatment. As I am a MELT instructor I am partial to that method and have seen good results.