How can I extend my kids' visa exemption in Thailand after 30 days?

The user seeks information regarding visa extension for their children arriving in Thailand on a visa exemption. They want to know the best time to apply for the extension, how long the extension will last, and whether an appointment is necessary at the Khon Kaen immigration office. Responses clarify that the extension adds 30 days to the current stamp and suggest applying 30-45 days before the current expiration. Making an appointment is recommended but not mandatory.