Is children's use of smartphones dangerous? If so, how do you protect them?

This article, Quarter of UK’s three- and four-year-olds own a smartphone, data shows shocked me as I thought I had gotten my twins smartphones too early, and that was when they were nearly 12 years old.
It reminds us that smartphones are becoming ubiquitous among even young children. Many parents have concerns that smartphone use could be dangerous to their children.  Do you believe your child's use of smartphones is dangerous? If so, how do you protect them from its dangers?