Is there any thing wrong in children seeing sexually explicit ads and movies?
Would you keep a razor sharp dagger near an infant? An infant does not know what the dagger can do to him. A child cannot learn from such an experience by itself; the elders have to take care that he does not hurt himself through ignorance.
A child does not know what sex is for. It will accept it as a normal event in life, even at that age, and it will definitely lead to actualize in its own life. Forward it to the rape games played by young children and the lurid dialogues of kindergarteners.