I had the same issue where the car would intermittently work, and then both driving and lights (and horn) were dead. From one minute to the next it would work and not work. I contacted Radio Flyer and they replaced the pedal assembly, and that seemed to help, but then the problem came back. It turn out to be a lose wire, and I've attached instructions they sent me that appear to have reliably fixed the issue. You probably want to skip down and start at Step 4 to remove the front right tire, then go back up to Step 1. The black wire they suspected was lose was indeed lose for me.
If this doesn't help, contact Radio Flyer, they were very responsive and helpful.