A group of students going to the dean sounds like the right path here. And there could be a number of ways they could go about this if it matters to them. Their money is at stake, not yours.
I would not advise going to the dean, unless the students also met with the DGS and chair first, and got no response. In my experience, deans are not usually that up to speed about how a department actually operates.
OP, are the students in the program self-pay? Or are they supported? (If self-pay, then is the program basically a cash cow, where student retention and "happiness" trumps all?)
Another question for the OP: Does the lack of rigor in the courses you mention eventually cause problems with when students take comps and do theses/dissertations? To me, that is the best reason to advance changes in how courses are taught.