Well, it sounds like a big part of what was wrong was that you gave them no warning about muscle soreness. If none of them trained with weights seriously before, that may have been quite an unpleasant experience for them.
They know the soreness was caused by the training, so saying "it would be so much worse if you don't work out", may not effective? You have to put yourself in their shoes, in order to be a really good trainer.
Sully's BBRx has a great section on training novice masters that can't squat. Some of those ideas probably would have served you well, and may in the future. Best of luck.