I read somewhere that small children see much more than adults, and supposedly can understand the speech of animals, cats or dogs. Perhaps this is a mockery of this theory or agreement with it.
It's not just that this is baby talk. Why is Cleveland's connection with the main characters mentioned? Just plainly compare the list of achievements of Peter and Lois, or Mag, Chris, the same Cleveland. With all his genius, Stewie is a small child for them already because Peter himself is a genius, Death is his friend, he hacked the matrix for a long time, constantly breaking his necks, exploding, torn to pieces, fighting with dragons, taking things from nowhere - besides, mind you, not enough any family guy is capable of such a thing. Yes, Stewie is a genius, but he was born from the seed of Peter, just as Lisa is no better than Homer just because she is smart. All Stewie's actions for them are like the actions of a child, simply because they have gone through all this more than once and not only, but he is just beginning to learn life
Love Blactually clarifies this question:
Stewie: I'll talk Loretta, let me talk to her
Brian: No, I think I can Better, she’ll only understand your words in general terms
Stewie: What are you? Isn't she one of the out-of-family people who understand me?
Brian: No, I think she's close enough to the main characters because of Cleveland, so that would be weird
Stewie: Are you sure?
Off-camera voice: Hey, we're filming!
Stewie: Oh, I'm sorry!
By the way, this is not entirely true.
In episode 12 of season 8 titled "Extra Large Medium", Chris, lost with Stewie in the woods, starts a dialogue with him about who they will become and what they will do after they get out of there.
By the way, they continued to communicate even after they were found and they returned home. It is a pity that only in this series.
Watch episode 1 of season 7 from 13:50 ͡ ๏ ̯͡ ๏ ٩ (̾ ● ̮̮̃̾ • ̃̾) ۶ ٩ (● ̮̮̃ ● ̃) ۶ ٩ (-̮̮̃-̃) ۶ ٩ (͡ ๏ ̯͡ ๏) ۶ ٩ (-̮̮̃ • ̃) ۶ ٩ (× ̯ ×) ۶ (• ̪ ●) d -_- b ♪ ♫ | ̅̅ ● ̅̅ | ̅̅ = ̅̅ | ̅̅ ● ̅̅ | ♫ ♪
Of all the characters, Stewie is the only one who takes Stewie seriously, the talking dog of Family Guy. Brother Chris also treats Stewie as an equal, but does not attach any importance to it. The rest are not inclined to see malice in his pranks, do not notice its development. Stewie himself demonstrates complete disdain, even contempt for those around him: he calls his father “fat”, “dumb”, “damn pervert”; mother - “witch”, “otter”, “dirty creature”, “stinking toad”, “dishwasher”; sister Meg - "pig"; brother Chris - "fat brother"; Brian - "drunkard"; neighbor Quagmir - "a block for whipping." At the same time, the attitude of adults towards Stewie remains kind and caring, they simply ignore his remarks (with the exception of Brian). One gets the impression that they are all just hearing the "baby talk of a baby."
As you know, Stewie is only a year old (we learn about this in the episode "Love Blactually"), and at this age the speech of children is not very clear (many begin to speak only their first words). It can be assumed that all the characters do not just ignore him, but hear childish babble, which they do not understand, but which has been translated for the audience.