Yeah kids often need a lot of supervision kinda a primary reason to get a babysitter.

I... I don't know what you were expecting especially with a fire hazard like a heater around.. this is literally what those big yellow warning signs on them are for

A fire wouldn't have started that quickly, especially if the item on the heater was wet, unless you completely weren't paying attention. How old is this kid, anyway? Surely he should know better than to put something on a heater. But regardless, sounds like you weren't properly supervising, which would make sense because you admitted you thought the job would be easy. So you were totally not paying attention, right?