Contract fraud requires that a representation was made, it was false, the defendant knew at the time that the representation was false, and intended that the plaintiff rely on it, in addition plaintiff relied on the misrepresentation and suffered harm as a result. In your scenario, the child does not make a representation, so failure to ask is the responsibility of the plaintiff. In general, there is no duty to speak, except in limited circumstances such as involves a serious defect or serious risk of injury. The plaintiff therefore cannot sue for damages, though if the minor disaffirms the contract in a timely manner, most states will require returning the goods / consideration in their possession. Also, certain contracts can't be disaffirmed by a minor.
Criminal prosecution for fraud is highly unlikely. In the US, most criminal fraud statutes are limited to specific kinds of fraud such as real estate fraud, vehicle fraud, check or credit card fraud, identity fraud, forgery.
