Before the Cold War, the United Nations was supposed to be a body that would be able to maintain peace in the world. It was supposed to represent the consensus of all nations. At the end of WWII, there were no formal rivalries between any major countries and so it was hoped that the UN would represent all countries in the world.
As the Cold War got started, this fell apart. The world started to split up into the Communist bloc and the US and its allies. This led to the UN being unable to do anything major because the two sides would never agree. So instead of being a major peacekeeping organization run on consensus, the UN had to be content with doing minor, noncontroversial things like UNICEF.