May be you have said too much already and even an apology would make matters worse.
Step back and leave your S & DIL to sort things out.
Also when you say "harsh" may be you need to define what "harsh" is in your view. You say it's not physical so that's one less thing for you to to be concerned about.
If it's about volume and tone of voice, children do get used to parents use of voice and somehow know when and how to tune out/disregard what's been said. It may sound "harsh" to some people but a child can normalised it.
If "harsh" takes the form of belittling or shaming your GC then you have to trust your S to get involved which he seems willing to do.
We as GP's don't always see is how many times the GC has been asked/told to, do/stop, whatever it is. So unless you are present in your S's family 24/7 you get a snap shot of their life, therefore , your information will be somewhat unbalanced.
Also what sounds "harsh" you may be heard by others as a parent just being assertive, authoritative, frustrated, exacerbated, worn out, sick of saying the same old thing AGAIN.
Your understanding of the your DIL's "harsh" could have other interpretations/ misinterpretations and it really is for your S, DIL and their children to find a way through their family life.
It's taken me so long to type this I'm sure others have said much the same as I have so apologies for any repetition.
Or don't you trust your Son to take care of/ stand up for His children? If you DO trust him, let him get on with taking care of them.