What do Kidneys do?

Your kidneys make the waste liquid your body has to get rid of.
You have two kidneys, one on each side of your spine, behind your stomach.
They are shaped like beans and are nearly as large as your heart.
Your kidneys filter your blood as it travels through them.
Where Are Your Kidneys Located? - The Human Body for Kids
Kidneys take out waste products that your blood has carried away from all the cells.
One of the main waste products is called urea. This is made after your cells have broken down substances called proteins. 
Kidneys also take away some salt and water from your blood if it contains too much of them. Your body must keep a balance of salt and water. Keeping this balance is the responsibility of the kidneys.
Your kidneys then get rid of the water and waste products by making droplets of a liquid called urine. 
The urine passes into your bladder through two tubes called ureters. 
Your bladder is a bag made of muscle, which stretches as it fills with urine.
When you decide it’s time to expel the urine, the bladder squeezes it out through a tube called the urethra.
Where Are Your Kidneys Located? - The Human Body for Kids