Yes, the world is a messy, sinful, rotten place at times, but that's why we need to be PARENTS. You are in control of what your child is exposed to, to a very large degree, especially when they're little. That is the time to fill their lives with wholesome, godly influences. It's YOUR job to monitor what they watch on TV, what music they listen to, and who they spend time with. Don't just tell them what NOT to do, give them fun alternatives! If they can't watch secular TV, give them fun, age-appropriate Christian cartoons and movies to watch. You can also find tons of things to watch that aren't "Christian" in label, but are harmless as well as educational (science shows, nature shows, documentaries). For example, my son absolutely loves shows about things exploding, or natural disasters, so we buy lots of science-y stuff for him. There's lots of music out there that he isn't allowed to have, so we do OUR part as parents and seek out awesome Christian music alternatives, and he loves it.
You can't control them forever, and you can't watch over them 24/7 (especially once they start school), but you can make sure that their home environment is a safe, godly haven, where the truth permeates their minds and hearts, and they can be "deprogrammed" after spending all day in the secular world. Remember, our job isn't to shelter them forever (though we DO need to do that to the best of our ability and what is reasonable), but to equip them to venture out into that messy world on their own someday.