Motherese is a technique parents use to facilitate language development, but the big gun is READING to your child.
In Motherese, the mother (dad) repeats and/or slightly expands upon what a child has just said, using, perhaps a different but related word in the process.
For example, the child may say, "I want to eat." The parent may reply, "Oh you want to eat--You're hungry."
A huge source of vocabulary, of course, is provided through the reading process! Show me a child whose parents read to him/her every night (starting at age 3 mos.) and I will show you a child with a very lusty vocabulary (not to mention an impressive grammatical competence).
Television doesn't do it, because the language on television is more often than not a simplified discourse.
This seems particularly true in the abundant mindless cartoons and sitcoms, which typically provide a meager vocabulary in comparison to books.
NOTES: See an example of Motherese.