Honestly, if possible, it is best.
I taught private classes of 4 year olds for years (not Gymnastics)
And having parents "In" the class environment can very easily ruin any class that involves any seriousness. If it is just a play-class, no problem. Makes no difference, but if it is something that involves anything more, then what happens is you end up with Children running to their parents every 5 seconds, any time they have to wait (if there is waiting for turns) or any time that anything happens that they don't 100% like.
Obviously the best scenario is a "viewing" area, e.g. a seated area slightly away from the learning area, of a window that you can look through etc.
What we used to do was to have a "no parent" policy, but we would film (with permission from all parents) sections of the class, so that the parents could see what their children were doing and the progress they would making. We would also frequently invite the parents in after the class, or at special occasions, so again - they could see.
Now as a parent, I still think this is a good balance. If I am there, I'm a distraction to my son, and a person to run to if the teacher wants them to do anything hard. I like it when there is a window, or videos, but I don't want to be in his class for any of the lessons.