BABY TALK is talked to babies only by adults with baby minds. Never talk down to your children; talk up to them at all times. Baby talk retards their progress in talking, and in learning to correctly pronounce and enunciate words. However cute it may sound to hear their first imperfect utterances, never encourage them in these imperfect utterances. Don't dwarf their speech in this way.
I do not believe in drilling a child in an effort to force correct expression, but if the child must be drilled, it is better to drill it in correct expression than in the incorrect expressions that constitute baby-talk. I have seen adult sisters who never got over their baby talk and who spoke to each other and to their mother in the crude imperfect way of a prattling child. This is a mental handicap I urge you not to place upon your child.
Let the child hear chiefly good language, observe good language and, thereby, cultivate good language. Give him a chance to develop rightly from the start and you and the school will not have to work so hard later to build him all over again--a work you are likely to fail in.