Why are my batteries getting hot?

[Sammus]

I can't imagine a servo dying would cause it. I'd put another vote on the battery not holding up any longer. Hmm but if that were the case you'd think it would be gradual.

Have any settings been changed on the ESC? perhaps reset it to stock (and upgrade to latest firmware if you havent). Maybe an accidental radio program (could be very hard to do... how young is your son? :P) could have set the timing high or something? Or could he have taken the initiative to gear it up himself? Or maybe he's just driving it harder?

All long shots I know, or possibly obvious things to think about (depending who you talk to), no harm in bringing them up though I guess...