I got an item called Childs Heart, but it is on the left side of the screen (opposite of where Q items appear) and I can't figure out how to use it. What does it do and how can I use it?
It's a trinket. Trinkets are passive, so it will be active as long as it is visible at the top left of your screen. You don't need to do anything special to use it other than pick it up.
No one knows for 100% certain what it does yet, but people think that it increases the likelihood of hearts dropping from killing enemies or clearing rooms.
Once people know for sure, you'll certainly be able to find the answer on the BOI wiki: http://bindingofisaac.wikia.com/wiki/The_Binding_of_Isaac:_Wrath_of_the_Lamb