B/t the lobes of liver is a rounded pouch gall bladder is located
The frog's urinary bladder is located in the lowest part of the body cavity.
Frogs do have bladders, but I don't think there is such a thing as a "small bladder" in a frog. I suspect that the asker may be referring to the "gall bladder". If so, please see the related link below which shows a frog's gall bladder, below its liver.
the organ that is underneath the liver in a frog is the gall bladder
The bladder located underneath the liver of a frog is known as the gallbladder. This small, pouch-like organ stores bile produced by the liver, which aids in the digestion of fats. In frogs, the gallbladder is typically greenish in color due to the presence of bile.
The frog's bladder functions to store urine produced by the kidneys before it is excreted from the body. It helps regulate the frog's water balance by retaining or releasing water as needed.
The Gallbladder is located underneath the liver and stores the bile of the frog.
It is similar to the function of the gall bladder of a human. They both are used to store the bile, a digestive juice, that is created by the liver. When needed for digestion, the gall bladder secretes a small amount to aid in the breaking down of the food (specifically the emulsification of fats).
The urinary bladder stores the urine in the frog.
It's how a frog "croaks" or makes it mating call.
urinary bladder
The storage of water so in dry periods a frog will still has a watersource. The bladder is also used as a defence mechanism; if a frog is handled the frog will spill his urine to distract the predator.\ The bladder is also use to filter urine