I'm assuming it's the fact the most common cause of arrest is asphyxia? So, therefore more oxygen is required I believe.

Just completed first aid course and have learned 5/30 and then 2/30. Didn’t really get this 15/2 though.

I think the main reasons for smaller ratio is because infants/children are 1) smaller, so don't need as many compressions to get blood round the body, and 2) because their lungs are smaller they need more oxygen. I'm an adult nurse and we've always done 30:2, so I'm just looking around at paediatric CPR!

according to the PALS guideline from 2015 a ratio of 30:2 should be used in both child and infants when there's only 1 rescuer available. If there are two in the scene then 15:2 is recommended.