There are many reasons for children's language retardation, including many pathological and non-pathological factors. If the reason is not clear, you can take a language test. If it is only the function of the pronunciation organs, it is generally possible to correct pronunciation directly to obtain accurate pronunciation.
Of course, the process of language correction needs a profound knowledge base and rich experience, especially for older children, who may have psychological problems and need a stable language environment and psychological environment.