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?
What does the Childs Heart do
the-binding-of-isaacwrath-of-the-lamb
Related Solutions
Sadly, no. There is no in-game list of what the items do. You can select Collection from the main menu to see a list of the items you've found thus far, but even that won't tell you what each item does.
The best source for item information I've found is the Binding of Isaac Wikia. It has a complete list of the items, as well as a description of what each item does.
For every enemy you kill, there is a chance that your damage will increase. That chance is high, but not 100% - I killed six flies and got four damage increases. It's also possible that each kill actually increases your damage, but the value that it is increased is not always enough to display another bar on the damage stat.
The damage increase is only in effect for the current room, though it is possible for the game to display an incorrectly-increased damage value on rooms after leaving the room.
Best Answer
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