see this article from mishpacha magazine for information on child & speech development – speech development
Speech development is something that concerns every parent. We often hear our children lisping or making other speech mistakes. There are many reasons why this can take place. In Israel, many times parents will give their child a non anatomical pacifier which can cause the speech to be altered. The bones of the face actually grow in a distorted manner as a result of this.
Oral habits like thumb sucking and finger nail biting can also be a factor in oral and articulation development. The bottom line is try and keep you children from developing these habits in order to prevent such problems.
for more info see: Speech sound development – Jackie Myland Seltzer, Speech-Language Pathologist