I use the AACN's Core Curriculum for Pediatric Critical Care. It's great.
There are only a few rhythms that kids get... sinus tachycardia, sinus bradycardia, premature ventricular or atrial contractions (atrial being more common), junctional ectopic tachycardia, supraventricular tachycardia, ventricular tachycardia and asystole. Pulseless Electrical Activity is not technically a rhythm change. Google images for each of these and see if you can pick out the differences from sinus rhythm. Another source for learning is eMedicine. They have an ECG of the month feature.