My niece has just stayed in hospital overnight for her oxgyen level to be checked but her heart rate kept going down to 79bpm is this normal for a 4 year old girl or is it got something to do with her heart problems that she was digonised with when she was 2 years old

Normal heart rate for children ages 3-5 years old is 80-140 bpm (beats per minute). One or two beats above or below most likely are of no concern.