Free web-based resources:
§ Life in the Fast Lane - EKG library
§ ECG Maven (case-based quizzes, linked by previous person too)
Peds-specific references:
https://www.bjaed.org/article/S2058-5349(20)30059-7/fulltext
https://www.ems1.com/ems-products/cpr-resuscitation/articles/introduction-to-pediatric-ecg-8eYPsnZOhCjQI9Hp/