Re-reading the OP, to answer the questions of while children pick up new languages easier and a foolproof method.
As I stated above, muscle memory plays a big part in speech. The mouth muscle memory in young children haven’t fully developed to certain speech patterns and others (family and friends) are quick to correct incorrect pronunciation.
Which brings us to a foolproof method. Total immersion as I mentioned above. When we’re young, we’re more open to corrections to our speech and have a desire to speak like an adult. If you hear an adult say “abble” instead of “apple”, you’ll let it pass since it’s still intelligible. However, if a child says “abble”, they’ll be continually corrected until they pronounce it as “apple”.
A couple more examples of how important muscle control and memory is for fluent speech.
When someone has a condition that causes them to not be able to speak for an extended period (e.g. a stroke), they sometimes have to go to speech therapy to learn how to speak again or use a different set of muscles to be intelligible. When you go to the dentist and get a mouth full of novacaine, you have to force yourself to use different muscles to speak.
And in order for a ventriloquist to sound like they’re speaking fluently, they substitute sounds and letters for those that usually require lip movement.
"First off, you need to be capable of saying some of the letters of the alphabet, you’ll find it easy to say some letters with ease, letters that don’t require you to move your mouth. Letter’s such as “A,E,I,O,U” the vowels. If you tried saying the whole alphabet by yourself you’ll find letters that you can say fluently and letters that you have to sneak by, I have found that those letters would have to be “B,F,M,P,Q,V, and W”. These letters requires you to move your lips and make them touch each other for their unique sounds.
The key to saying these special letters are simply by “substitution”. You see ventriloquists substitute their (upper) lips with their tongue, so in order for you to say the letter ‘b’ you’re gonna apply pressure to the back of your front teeth with your tongue.
In order to say ‘M’ – you’re gonna do the same thing and substitute and go for the ‘N’ sound
In order to say ‘P’ – same thing and substitute and go for the ‘T’ sound
In order to say ‘F’ – you’re gonna hiss through your teeth like a ‘th’ sound but still your tongue is gonna be touching the back of your teeth
In order to say ‘V’ – you’re gonna substitute ‘th’ sound
In order to ‘W’ – substitute the “ooh” sound as in “boot”
When you start to perform in the ventriloquism field, you’ll find out that as long as the audience could interpret what the ventriloquist and the ventriloquist dummy are saying by making out ‘most’ of the word, you can get away with it, but think about whether you want your audience to think about what the dummy is saying all the time."
Edit: https://theartofventriloquism.wordpress.com/