I think EBD's unrealistic view of life comes primarily from the fact that she had no siblings during the majority of her childhood, her brother having died when she was quite young. That said, children in that era didn't have the freedoms that modern children do and any fighting would have been done behind closed doors in the nursery. There are a couple of instances in the later holidays books, for instance in Future:
“Have it your own way, but don’t dither. I’m so thirsty, my tongue’s hanging out of my mouth. It’s a pity,” [Jo] went on with a wicked look at Con, “that we didn’t ask Irma for one of those two last bottles of blackcurrant wine of hers. I’m sure she’d have loved to let Con have half of what was left, seeing she’d already had quite a dose of it.”
“If we weren’t in Minnie,” Con said severely, “I’d tell you exactly what I think of you. That business wasn’t my fault!”
Mike’s eyes gleamed. “What did it taste like, Con?” he asked. “Oh, go on! You must know. I’ll bet you licked your lips clean, anyhow.”
“Well, I didn’t,” Con said coldly. “I’m neither a dog nor a baby boy –”
It was just as well that Mike was fenced in by Roddy. He did jump to his feet with a shout of rage, but Roddy stiffened and he could hardly climb over him. In any case, his father ordered him sharply to sit down.
“If you can’t take teasing at your age, the Navy’s off, where you’re concerned,” Jack added. “You began it yourself, anyhow. Pipe down!”
Mike knew better than to argue. He sat down sulkily and, as they had entered the town, there was plenty to distract his attention. Joey, leaning across and speaking in an undertone and fluent German, told Con to leave him alone.
“You’re years older than he is. You’ve no need to mind his cheek. Let him alone, Con!”
Con blushed. “Sorry I didn’t mean to get his goat.”
There is also the simple fact that, in many books, children do behave better than they really do. It's probably because, as you say, it is difficult to get things done if the kids are always misbehaving. For instance, look at the Bobbsey Twins, the younger set of whom are six but almost never behave like any six-year-old I ever met! And the twelve-year-old set of twins act more like eighteen or twenty IMO (in fact I have just checked and they're four and eight respectively in the first book, so it's not just EBD who can't keep ages straight!).