What Patricia said: Your kids love it and are doing well, and it was not a normal day. Yes, you were the only parent in that one classroom at that particular time, but if there were any other events going on at any point that day because it was parents' day, the kids were revved up and thrown off kilter. I might not judge a classroom based on any special day.
Though your child said it's always like that, bear in mind, kids tend to view any "pipe down" kinds of talk from a teacher as "yelling at us" (even if it's not) and also sometimes turn a few instances of the class being rowdy into the idea that the class IS rowdy by definition and every day is like that all day long. I am not saying your kid isn't being truthful; I'm saying that when I used to ask my daughter, for instance, if the class was loud or disruptive, I learned quickly that the answer didn't always correlate with what went on most of the time -- being a kid, she tended to recall rowdiness (and any shushing by a teacher) as being a bigger deal than it was.
Also, if you delve into it you might find from talking to your child that it's the same few kids every day who have issues, make trouble, can't move on to the next thing, etc. That was the case in my daughter's fifth grade class a few years back. One kid in particular, and two to four others who tended to get inspired by his antics, were the main source of things. I know that the parents did work with this one boy and got him counseling and other help and by the end of the year the boy was much improved. Kids like my daughter, a rule-follower like your kids might be, found this boy frustrating, but it was a good chance for her to learn some patience--and to learn to stand up for herself and speak up when this boy was annoying people to the point they could not do their work! That's one thing to work on with your own kids-- they have a right to a classroom where they can actually work, so practice with them how to respond when their peers are disruptive.
You mention that the school does not use parent volunteers. I assume you mean in the classroom for main subjects? Are your kids somewhere beyond earliest elementary grades? Once kids are past early elementary, about K through 2 or maybe 4, teachers need and want less and less parent involvement directly in the classroom, but that does not mean you can't volunteer in other ways at school and get a very good feel for how things are overall, not just in the classroom. Art, gym, extracurriculars, PTA events and PTA-sponsored, ongoing programs, tutoring during students' free periods, copying in the office, and (my personal favorite which I did for years) volunteering in the library where usually they always need help -- doing a combination of those both gets you known by teachers and the front office, and gives you a larger picture. And yes, try to get to every class event or party that parents are allowed to attend. It's not the same as watching a teacher teaching math or English, but it does give an overall feel for the school and how well or poorly run it is.
Then you'll have a lot more information (and frankly more standing with the teachers and front office) to say something if you feel that there really is a pattern of disruptive kids and teachers who can't quite control them.