If your child has a speech impediment you need to be a good model for your child. Talk at a slower rate and speak clearly. If your child is working on the k sound and he/she says tow for cow. You should say yes that is a cow, with a slight emphasis on the k so that your child is hearing the sound produced correctly.
It is important to read to your young children, this will help increase their vocabulary skills and grammar. Use a variety of vocabulary when speaking to a child. If your child says, "That is a big dog." You could say, "Yes, that dog is huge."
Your Speech/Language Therapist should provide you with activities to work with your child at home. To help your child to be successful in their speech/language skills is to work with them at home!