Beginning at age 4 years old a child will typically need to urinate every 20 to 45 minutes depending on how much fluids they drink.
Children at 4 and 5 years old typically urinate in small amounts since there bladder is so small.
As the child ages to 6 and 7 years old and above they typically only need to urinate every hour or hour and a half.
But again all children are different and may need to urinate more often or less than other children of the same age.