My kids often get a hoarse voice with a sore throat or cough. Has your child had any other symptoms?? With a quick onset, it may be a viral episode. Any fever?
Not certain where you live but I know here in the US many children are sick with similar, it may be allergies, check out the environment in your area it should tell you online where your air is unhealthful , an anti histimine may help , best ask the doctor for help in treating your child.good luck