I'm sorry you're facing such a frightening ordeal right now. I honestly don't have any suggestions because you don't know yet what's causing the enlargement or how severe it is. Sometimes as adults the enlargement can be reversed to some degree with medications. I'm not sure with little children.
Did you know there's a pediatric cardiology forum here? I'll add the link but if it doesn't work, just click on "Forums" at the top of the page and scroll down till you find Pediatric Cardiology along the left side. I was just there and someone else posted a message about their 3 year old son having an enlarged heart. Parents there will share their experience and knowledge.
Take care.
www.medhelp.org/forums/Pediatric-Cardiology/show/302