Writing a convincing simulation of child's speech is not easy. Besides sticking to primitive vocabulary, the challenge is to ascertain which types of phonological processes contribute most to the perception of childishness, while at the same time maintaining legibility and a pleasant tone.
As a rule of thumb, we will avoid phonological processes which replace sounds with common ones like /d/ & /t/ which often render words completely illegible. Thus, most of the stopping processes ahould be avoided when legibility is important.
Velar fronting, especially 'ng' -> /n/, can often be used to good effect, as can palatal fronting. However, they can destroy some expressions: 'That tin is a saddy dod," used to be "That thing is a shaggy dog."
The easiest to include is the gliding of liquids. Most pople are familiar with its sound -- it replaces the common sounds /l/ & /r/ with the uncommon /w/ (or occasionally 'y'). Most can understand when a "wowipop" (lollipop) is asked for.
Another good one is final consonant deletion. Quite effective on plurals and other non-vital trailing consonants. "The las cookie is perfeck," (The last cookie is perfect.)
It seems there are important exceptions to avoiding 'stopping': voiced 'th', unvoiced 'th', and /v/, which become /d/, /t/, and /b/. Again, it seems familiarity renders legible "dat tiny ting ober dere." (that tiny thing over there.)
Consonant harmony and weak syllable deletion are also good candidates. They relate to the whole word instead of a single phonem, and are also fairly familiar. Most could understand "the choclate is mime." (the chocolate is mine.)
Of course, we can always tack on the classic lisp for good measure, replacing 's' and 'z' sounds with 'th'.
Let's look at an example, and add one phonological process at a time.
"Hi, I'm Samantha. I like eating lollipops and chocolate very much. Roses are pretty flowers and smell nice. I have a kittycat, her name is Lucy."
(+gliding of liquids)
"Hi, I'm Samantha. I wike eating wowipops and chocowate vewy much. Woses are pretty fwowers and smeyw nice. I have a kittycat, her name is Wucy."
(+stopping 'th' & /v/, 'ng' -> /n/)
"Hi, I'm Samanta. I wike eatin wowipops and chocowate bewy much. Woses are pretty fwowers and smeyw nice. I have a kittycat, her name is Wucy."
This is a good place to stop: the voice is clearly childish, but the meaning is still clear. One can go farther, but I personally think legibility gets much worse.
(+consonant harmony, weak syllable deletion, and final consonant deletion)
"Hi, I'm Samanta. I wike eatin wowipop and chocwat bewy much. Woses are pretty f'owers and smeyw nice. I have a tittytat, her mame is Wucy."
Sounds pretty good, huh?
Let's try adding a lisp, just for fun:
"Hi, I'm Thamanta. I wike eatin wowipop and chocwat bewy much. Wotheth are pretty f'owerth and thmeyw nyth. I have a tittytat, her mame ith Wuthy."