The reporting is so poor, I have no indication regarding the child's known allergies, etc...Perhaps Delta cabin crew will know more. Further, I'm not claiming she had an allergy...Only that anaphylaxis can drop BP and cause a cardiac event. So, if you simply attempt to treat the heart, you will likely not stop the deadly reaction.
But, I'm less here to inform and more here to learn. Hopefully more data will come out and perhaps someone on the flight will answer more details.