A recent study has shown that kids develop stronger vocabularies when read to (surprise!) and talked to by a real adult, not one on TV.
…the research team found that with every hour per day spent watching baby DVDs and videos, infants learned six to eight fewer new vocabulary words than babies who never watched the videos.
I will toss my opinion into the fray and add that I think the best thing you can do for your baby/toddler/child is to read to them every day and talk to them. It doesn’t matter what you are talking about. Give them a blow-by-blow account of how you are fixing their breakfast. What matters is that they are listening to your voice. They are hearing your intonation and gaining new vocabulary. Ask them questions and then answer them yourself if they are not old enough to carry on a conversation. I don’t think all TV is bad, but I do try to limit The Boy’s TV watching to a half an hour per day. And to be honest, he usually loses interest after about 10 minutes anyway.
As a former teacher, I can not stress enough the importance of encouraging kids to read and reading to them. I could usually tell, by the end of the first week of school, which kids were read to at home. Most of the time, they were more focused and willing and able to do grade level work.
As your kids grow, try to set aside at least a little time every day to read with them. By talking and reading to them, you will see their vocabulary grow rapidly every day!