How many calories would someone who weighs 38kgs/83lbs burn in an hour of indoor cycling?

Depends: Speed of cycling, resistance of the cycle, and style of upper extremity grasp can effect cardiac challange and caloric burn. Please consider your ideal weight for height, as the weight you report is similar to that of a child. Cycling for fitness would mean that you maintain adequate muscle and bone mass. Replace calories burned with adequate protein, calcium, vit d and necessary hydration.