Question: What is (are) Kidney Stones in Children ?
Answer: A kidney stone is a solid piece of material that forms in a kidney when substances that are normally found in the urine become highly concentrated. A stone may stay in the kidney or travel down the urinary tract. Kidney stones vary in size. A small stone may pass out of the body causing little or no pain. A larger stone may get stuck along the urinary tract and can block the flow of urine, causing severe pain or blood that can be seen in the urine.
Source: NIDDK

Question: What causes Kidney Stones in Children ?
Answer: Kidney stones can form when substances in the urinesuch as calcium, magnesium, oxalate, and phosphorousbecome highly concentrated due to one or more causes: - Defects in the urinary tract may block the flow of urine and create pools of urine. In stagnant urine, stone-forming substances tend to settle together into stones. Up to one-third of children who have stones have an anatomic abnormality in their urinary tract. - Kidney stones may have a genetic cause. In other words, the tendency to form stones can run in families due to inherited factors. - An unhealthy lifestyle may make children more likely to have kidney stones. For example, drinking too little water or drinking the wrong types of fluids, such as soft drinks or drinks with caffeine, may cause substances in the urine to become too concentrated. Similarly, too much sodium, or salt, in the diet may contribute to more chemicals in the urine, causing an increase in stone formation. Some doctors believe increases in obesity rates, less active lifestyles, and diets higher in salt may be causing more children to have kidney stones. - Sometimes, a urinary tract infection can cause kidney stones to form. Some types of bacteria in the urinary tract break down ureaa waste product removed from the blood by the kidneysinto substances that form stones. - Some children have metabolic disorders that lead to kidney stones. Metabolism is the way the body uses digested food for energy, including the process of breaking down food, using foods nutrients in the body, and removing the wastes that remain. The most common metabolic disorder that causes kidney stones in children is hypercalciuria, which causes extra calcium to collect in the urine. Other more rare metabolic conditions involve problems breaking down oxalate, a substance made in the body and found in some foods. These conditions include hyperoxaluria, too much oxalate in the urine, and oxalosis, characterized by deposits of oxalate and calcium in the bodys tissues. Another rare metabolic condition called cystinuria can cause kidney stones. Cystinuria is an excess of the amino acid cystine in the urine. Amino acids are the building blocks of proteins.
Source: NIDDK