A federal proposal that would restrict the kinds of foods marketed to children may soon be substantially changed to address the concerns raised by the food and beverage industry, which has aggressively lobbied against the plan for months. In a statement submitted to a House panel on Monday, a federal regulator deeply involved in developing the voluntary guidelines said the government is taking a “fresh look” at its proposal and rethinking some of its most hotly contested aspects, including how it defines “children.”