The Electrocardiogram is a graphical representation of the electrical work of heart muscle. It is useful in looking for heart rhythm problems, heart enlargement and other cardiac problems.
The Holter is a device that is worn and attached to child's chest to record the heart rhythm over 1-2 days.
The Event Monitor on other hand operates wirelessly and is smaller than the Holter. It is used over long periods to record the ECG when needed at home, and is activated during the 'event' by either the patient or a parent.