No one knows for sure how children learn to pronounce their mother tongue.
For centuries, people have said that children learn to pronounce ‘by imitation’ and have not looked further. This answer is certainly much too
To properly investigate the question, different aspects of pronunciation have to be distinguished - at the minimum:
Briefly, at Pronunciation Science we think that the best evidence now available supports the notion that a child learns the pronunciation of speech sounds as a result of his mother imitating him. (A mother imitating her child is something that happens very much more than a child imitating his mother.)
And children learn the rhythm and other timing effects of English because they have bodies that are different from adult bodies: smaller, but also with different physiological and aerodynamic characteristics. As a result of this, children have control their breath differently from adults. A child's distinctive speech breathing affects the timing of his speech in a varieity of ways.
For details, visit Piers Messum's website.