Could a heart rate of 200 beats per minute be sinus tachycardia?

Yes: But it will depend on the age of the patient and the circumstances. A neborn baby can easily have a hr of 200 when crying or even feeding; with vigorous cardio-vascular exercise a healthy young adult's hr can reach 200 BPM , but if that hr occurs at rest you must suspect an abnormality. Have yourself checked.

In a child, yes: In adults a hr of 200 is abnormal. In children the hr may reach 200 with vigorous activity or anxiety/distress. In infants the hr can often reach 200 with intense crying, or with fever.