I have seen this firsthand, obviously. I know that my own children were using words like children and neighborhood by the age of three. They spoke in complete sentences and understood how to have a conversation with an adult. Whereas my students struggle with relatively basic vocabulary, especially my non-english speaking children. Case in point... this past spring I was teaching the basics of multiplication. Multiplication begins by understanding that it is simply repeated addition: three pairs of socks would be 2 + 2 + 2 = 6. We do many activities such as: how many legs would be on the playground if there were 15 kids? or If there are 24 feet in the pool, how many people are there?
One particular problem was to tell me how many toes there would be on three people. I "ensured" students knew how to complete the problem, then went about helping students with the addition. After a while, it became obvious that two students were struggling. When asked to show me their work, they went about showing me 2 + 2 + 2 = 6. I asked them how many toes they each have. They both pointed to their feet and replied "two." So, basically it wasn't the math that they were struggling with at all, it was the vocabulary. Neither one of them understood what "toes" were. Another student asked later what fingers were. The lesson here is: DON'T ASSUME ANYTHING! My assumption was that all of my students would know their body parts. I was mistaken. Don't make the same mistake I did.
This is a great article about the thirty million word gap:
I have also included a flyer that you can print and post in your classroom. I think it is valuable.
Here's the link for that: http://alamosa.k12.co.us/evans/assets/files/ssanchez/Word%20Gap.pdf