Are there any laws protecting minors from those "free-trial offer" mail order scams?

There are in the sense that minors generally can not enter into contracts, and therefore contracts entered into by minors are generally voidable.  Lemon laws vary from state-to-state.  If your child signed up for something like this, I would contact the company and explain that it was a child.  They should be willing to let you out of it.