@BobbiSueEllen has got a good grip on it there and to expand a little.
A lot of baby talk I see from AB's involves substituting letters (fwend for instance) to sound cutesy but while a lot of young children who are still learning to speak have trouble with certain sounds it is often in different places than many AB's using baby talk think.
You could try studying some speech and language therapy to get an idea of what ages children should be capable of pronouncing certain letters (some are quite surprising)
This document for instance is a very good guide on sounds and pronounceable letters as well as where and why certain letters may be missed/added/substituted.
Another thing touched on was pronouns and BobbiSueEllen was right there, especially with third person usage but I often see things like "my" being used a lot instead of "I" at first with phrases like "my hurt my knee", "My wants an appaw", "My needs a wee wee", rarely do they use "me".
They also often don't start getting tenses until later on so there is no separation between things like "I rode my bike" and "I'm riding my bike" it would be said more like "My ride my bike" whether they have done it, are doing it or want to do it. Same with ate, eating, to eat.
Then there are the S's, lots of S's get added to words like "I wants..." I needs..." and so on, I think much of this comes from them overhearing people talking about them and just picking up the words so if mummy says "Molly likes to play with Lego" when the child is then asked what they enjoy will reply "my likes Legos"
There is a lot of depth to how children learn to speak and what they are capable of at different points.
