Rugrat mispronuncations further explained
I know I've touched on this before, but I thought I'd try to explain it another way.
During the Germain era, the philosophy behind the talking babies was that they were perfectly capable of speaking, the adults just didn't know it. The writers never decided whether it was because they were speaking behind the adults' backs or because the adults simply couldn't understand their language.
But the key was that the babies were not limited intellectually by their cognitive development, they were limited by education. They were every bit as smart as the adults, they just had no education whatsoever aside from what they learned on their own through observation. This was what led to numerous misunderstandings, and these were among the key comedic devices for the series. The babies would draw false conclusions about something in the world in an amusing way, and their misunderstanding would often ironically turn out to be correct in a way.
The Rugrats didn't mispronounce words in this era, they mistakenly said the wrong words, or misunderstood the construction of a word, in a way that led to an amusing play on the word or expression.
For example, when Chuckie was talking about how nobody could make him stop wearing his diapers, not even the President of the United States, he said "B-Nited States" instead. This was not a matter of Chuckie not being able to pronounce the word, "United." It was the fact that while Chuckie had heard of the United States before, one of the things he remembered is that it started with a letter of the alphabet. He mixed up which one it was, hence "B-nited" instead of "U-nited/United."
Since Chuckie has never gone to school or received any kind of formal education, he has only overheard the name of the country, that there is a "President" of the country, and he has overheard some of the letters of the alphabet and recognized that the name of the country starts with one of those letters.
When Chuckie is narrating in "Blizzard," he says dates such as, "Septembruary 37th." This is because he confused the names for months he has overheard people mentioning, hence he says an amalgam month, and he does not know that there are no 37 day months. Again, this is a mistake that emanates from a lack of education, not a lack of cognitive development. Even when he said "Barch" to mean "March," it was simply him not remembering the proper name, not him being unable to pronounce it.
When Chuckie counts during Hide-and-Seek, he pronounces all the numbers just fine. He just says them out of order because he never learned the correct sequence. "Four, Seven, Twelve, Eight, Five, One, Seventeen. OK, here I come."
The problem with the newer writers is they mistakenly thought that the idea was that the babies were merely advanced for their age, hence they wrote their lines as though they were trying to imitate the limited way in which 3/4 year olds speak. They were trying to mimic a little kid's undeveloped speaking ability leading to struggles pronouncing words correctly. In the Germain era, the Rugrats had no difficulty pronouncing words correctly. A "diaper" was a "diaper." Because they weren't 3-4 year olds, they were babies the parents thought were mute who could actually articulate themselves perfectly. The only 3-year-olds on the show were Angelica and Suzie, and they pretty much openly spoke like adults, outside of their own educational limitations.
The new writers also projected that adult-created "cutesyness" onto the characters, leading to sickening faux-words like "diapees" coming out of the babies' mouths. This is the writers' demonstration that they're the type of people who use "baby talk" around their babies/very little children/pets and wishing that the babies and pets would actually talk the way they talk around them. "Awwww, you want a blankee? You need a diapee change? Are you sweepy, witto boy?" "My witto puppy, you're a good boy, yes you are. Yes you are."
In actuality, the cuteness from little children/pets in real life is unintentional. The puppy gets food all over its face getting into the pantry, for example. If a baby or small child ever uses one of those cutesy long-E ending words, it is only because an adult has taught them that's the word they should use.
The Rugrats misunderstanding things in the Germain era was naturally cute. They weren't trying to be cute, they were just falling victim to their limited experience. It was cuteness at a distance...the characters were trying their best but they were constrained by their circumstances.