Which condition is characterized by excessive thirst and frequent urination in children?

The condition characterized by excessive thirst and frequent urination in children is indeed Diabetes Mellitus. This condition results from the body's inability to produce sufficient insulin or effectively use the insulin it produces, leading to high blood glucose levels. The kidneys attempt to excrete this excess glucose through urine, which increases urine output and, consequently, can lead to dehydration. As a result, a child may experience excessive thirst as the body attempts to compensate for the fluid loss.

In contrast, while dehydration can also lead to increased thirst, it is more a result of loss of fluids rather than a primary condition that causes excessive thirst and urination. Acidosis refers to an imbalance of acid in the body, while hyperkalemia involves elevated potassium levels and does not typically cause increased thirst and urination. Therefore, Diabetes Mellitus is the most accurate answer for the symptoms described.