Ringworm is a fungal infection of the scalp and indeed can affect a large area. It has a characteristic appearance and can take at least a couple of weeks to heal. The hair loss is typical and should regrow after the infection is treated.
There is another infection that can affect the scalp but it is unusual in a child of six. This usually improves with ketoconazole shampoo. However you should continue with the selsum and griseofulvin for the rest of the course before making any changes.