My daughter is 1 year 6 months old and is complaining of pain when she urinates or feels especially when her diaper has been on her for a long time.

Probably vulvitis. She complains when the diaper is on her for a "long" time. The urine in the diaper is being held up against her genitals and urine can be very irritating to her skin. Let her sit in a bath tub with just clean water, no bubble bath, and after you dry her put some petrolatum on her genitals.