I'll throw out one that never seems to get mentioned when a variation of this question is asked:
"How to Read Pediatric ECGs" by Park. ECG content is about 150pp with ~another 100pp of sample questions. Does a very good job of covering basics of ECGs (much of which is universal) and things specific to pediatric ECGs (i.e. normal things that would be abnormal in an adult and abnormal things for age). If you're willing to read a small book on the subject this would definitely fit the bill. Alternatively the chapters on ECGs and rhythm disturbances out of his book "Pediatric Cardiology For Practitioners" would be a bit shorter. The whole book would be overkill for an ED doc to read, though it would be a good reference to have in the ED, esp. when you want to look up something a little esoteric on a patient you have time to read about.
I think this is a very worthwhile book as most docs who order peds ECGs (including FM, Peds, and EM) don't do a great job reading them. My experience, when I read them for them, is more over-calling normal stuff abnormal. This might help.
