Can a retired parent sponsor their adult child and grandchild for a visa in Thailand?

Yes, a retired parent can sponsor an adult child for a visa in Thailand, provided they meet the financial requirements, such as having a minimum balance of 500,000 Thai Baht. Although some embassies may prefer employer sponsorship, your father can support your application financially, especially since you work online. If you're considering activities related to Muay Thai, you should also clarify which visa type is necessary, especially if applying under the 'Workcation DTV' for remote work activities.