Our staff has been discussing any gaps in our curriculum from one grade to the next. For example, in one year there may not be much emphasis on a particular subject, but the next year the rigor of that same subject makes it difficult for students to keep up. For the most part, our curriculum matches up from grade level to grade level! However, writing can be a struggle in older grades and teachers cite a lack of correct oral language as a detriment to written language.
I suggest having a time in your house when correct language should be used, for example at the dinner table or in public. In an age of text language and quick, unedited emails, many students never learn to speak formally. Children should, of course, be comfortable with how they speak and be given a time to speak without being corrected. However, there is a time and place, from childhood to adulthood, when formal language is appropriate and necessary. From writing a letter to an essay, from ordering at a restaurant to interviewing for a job, people need to speak in complete sentences with correct grammar.
Learning that starts now. For example, if you hear your child say "'cuz" instead of because, correct him or her. Most kindergarteners don't know that these are the same word and one is actually incorrect. If you ask them a question, instead of replying "yeah," they should reply, "Yes, I would," etc. Again, I wouldn't correct your child every time. I don't want speaking to become frustrating to them or hurt their self-esteem. However, set a time of day or setting of your home or community when that speech is taught, and teach your child when they need to use it.
Hopefully, not only will this make writing easier as they get older, but your children will have another tool they need to become a successful adult!