The answer is yes. Because your foreign child is your dependent, and the child is in full time education in Thailand, you can “piggy back” on your child’s visa, converting your current non-immigrant visa to match your child’s long stay visa. please note that the immigration will ask you to show some kind of evidence of how you are supporting your child in Thailand if you are not currently working; be it money in a Thai bank or a regular income from your own country. There is not a specific amount and is at the discretion of the immigration office, so have your information to hand.