If I understand your question correctly, you are pertaining to when fergie sings "rock your body! C'mon c'mon rock your body!".
Then on your title itself, you've mentioned one way to do this. It's either a vocoded or autotuned vocal then either the pitch or formant was raised.
I also hear this sound abundantly on hip hop and R&B songs.
Examples:
"All I have" by Jennifer Lopez ft. LL Cool J
"like Toy soilders" Eminem
I'll mention 2 of the easiest ways to get this sound (no vocoders needed):
1st method: Easiest and most overlooked way to get this sound is to actually get a child/kid (or a female with a high pitched voice) to sing the part and add processing as desired. (like maybe add EQ for the telephone effect, reverb, chorus, or add saturation/distortion ).
2nd method: This is the most common way its done as it does not require a child and anyone who has access to a singer (or vocal samples) can do this. Once you have the vocal line you want sound like, as you said, a "baby" or a "kid", you can then raise the pitch in you DAW or raise the formant with a plugin.
RAISING THE PITCH: You can simply do this in your daw, just google a tutorial. You can also achieve this with a dedicated audio editing software like Audacity. Be aware that this method can sometimes alter the timing and audio quality of your vocal sample (artifacts such as cracks and pops may appear). TIP: If you have the vocals already in key, just raise the pitch to +12 semitones (1 octave) so you won't have to worry about the key of the song.
RAISING THE FORMANT: Usually pitch correction plugins have a formant control such as Autotune, Melodyne or FL studio's Pitcher and Newtone. You have to be careful about this as pushing the formant too much will make your vocals sound really chipmunky, robotic or even alien like. You may also need to raise the formant after raising the pitch, especially if you have a male vocal sound.
Again after choosing either methods, apply processing as needed. (EQ, compression, reverb, chorus, saturation/distortion etc.)
I hope I understood your question correctly and was able to help. Peace.
