While perhaps a little premature, the OP's quest for better understanding seems reasonable.
To share an EMS perspective, the large majority of pediatric cardiac arrests are caused by respiratory failure. That would be consistent with a systemic anaphylactic reaction causing airway compromise. Epi-pens may buy some time but if the airway occludes fully - which happens quickly with kiddos - the last hope is an intubation kit, and a passenger competent in its use. AEDs have made a huge impact on public health but won't help if oxygen can't get into the blood that the heart's trying to circulate. Alas, it simply wouldn't be practical for airplanes to carry the equipment and skillset for every foreseeable health emergency.
None of the above is based on any information specific to this case - a profound tragedy at any time, but especially this time of year.
