@ChaircaMiaow
There is a lot of myth and miseducation surrounding cardiac arrests, and AEDs (and defibs of any sort) are only useful in a small percentage of cardiac arrests (whilst defib machines will always be connected to the patient in any arrest the "shocking" function is not used in many cases, but the pads/machine double up as monitoring equipment).
So, Defibrillation is generally (I say generally as the human body seems infinitely capable of throwing curve balls) only of use in cardiac arrest where the cause of the arrest is a primary problem of the heart. In other cases of cardiac arrest, shocking a patient is not indicated (contrary to what Casualty might have you believe):
Hypoxia (lack of oxygen) so all you primary airways problems; choking, strangulation etc, and primary breathing problems; Asthma, drowning etc
Hypovolemia - Lack of circulating blood, Bleeding to death, severe dehydration, circulatory collapse in anaphylaxis.
Hypothermia (whilst this can lead to arrhythmia arrest in severe cases, shocking a really cold person is ineffective until they are rewarmed)
Hypo/Hyperkalemia
Hydrogen ion acidosis - acidic blood (ties in with lack of oxygen and is the end result of too little oxygen)
Toxins - poisoning, accidental and deliberate overdoses, ie heroin causing central nervous system depression, slowing or stopping breathing and resulting in hypoxia (obviously some drugs will have a direct result on the heart and in these cases my lead to an arrhythmia and subsequent cardiac arrest that may be shockable with a defib)
Tamponade - Where the space between the heart muscle and the hearts fibrous sac fills with blood, consequently compressing the heart so that it can't beat effectively.
Tension pneumothorax - where there is a "leak" from a lung, which fills the chest cavity with air and compresses the heart so that it can no longer beat effectively.
Thrombus - massive blood clot in the lung leads to respiratory collapse and lack of oxygen
All of these need the cause to be treated if there is to be any hope of saving the patient, an AED will not achieve this. Even if one of the above problems leads to an arrhythmia (incorrect heart beat), the cause of the cardiac arrest still needs to be dealt with to have any hope of saving the patient as they will just keep going back into a fatal arrhythmia.
Most paediatric arrests will be due to one of the above causes, apart from a tiny minority of children with underlying heart conditions (which may or may not be diagnosed).
When defibrillation is indicated and a shock is delivered promptly the results can be amazing, but a defib on a beach is only likely to be useful as a numbers games, ie lots of lots of people in a small area, if one happens to have a cardiac arrest due to a heart problem then it might be reversible with the AED (and the slight increased risk of an arrhythmia becoming problematic due to cold water immersion).