The accusation that you are making is both very serious and not simple to prove. People often make such accusations from a deeply biased perspective, and it's easy to get written off as "sour grapes." At the same time, serious misbehavior and manipulation of journals does happen.
I would thus suggest that the first thing you do is to gather some scientific evidence of the journals' scientific misbehavior. You claim that the editors preferentially publish their own students' papers, and that these papers are poorly cited. This is a claim that can be backed up by data analysis. Both academic genealogies and the journal's publication and citation records are publicly available records. Tabulate the behavior of the journal before and after the editors you are suspicious of took over. Compare it to the behavior of peer journals. Plot the data.
In doing this, you may discover that you are not actually correct in your accusations. It's easy for this to happen due to confirmation bias, and especially when we feel passionately about something. If so, then you have saved yourself a lot of trouble and embarrassment.
On the other hand, if you discover that you are correct, then you have everything in hand that you need to seek the reform you want. First, you'll need to be clear on what positive action you want to be taken: "don't do that" isn't very actionable. Do you want the editors to step down? Do you want papers retracted? Do you want something else? Then you can go to the editors (who might be unaware of their own bias), the publisher (who may or may not care), and to social media in general.
Remember, however, that the prior probability of such accusations is that of an angry and biased person or a crank, and you need to work hard to overcome that impression in every interaction. You will need to be very careful to hold yourself to work based on facts and clear, reasonable requests rather than passion if you want to get the results you seek. I might suggest Lior Pachter as one (possibly controversial) model to follow: he is both very aggressive and very precise, careful, and data driven in the way in which he calls out what he sees as significant scientific problems.