There is only one way to handle parents, and that is to make the first positive move. So, frankly, you are already behind if you are hearing from them through their students. It means that they don’t feel like they have an open line of communication with their kids’ teacher.
And that is your fault.
Parent outreach starts immediately
If you had a time machine, you would go back to the first week of school and follow my program: starting no later than the second day of school, I always called every single parent in the class and thanked them from for trusting me with their children.
This is a huge positive opening that pays dividends all year long in communication. If they have issues, they call me and we work it out. Simple as that. Nothing back-channel through the students.
These parents of yours don’t feel like they have a way to contact you for a discussion, so they work themselves up into an emotional state before finally bending your ear.
But you don’t have a time machine, so you’ll need to reach out now. The first time you hear something negative being said about your classroom, I'd call that parent and say:
“Wow, I'm hearing some things. Tell me more about that. Let's clear up what may likely be miscommunications.”
Then, going forward, step up your parent outreach by calling and emailing to share positives with them. (Hint: they love it.) You’ll be inoculating yourself for the next kerfuffle.