If you are only going to stay 89 days...then the tourist visa is perfect for you and your mother.
That will give you 60 days on arrival plus 30 day extension = 90 days stay.
You both will need COVID insurance to enter and stay...due to mothers age of 90 best option is to buy the COVID only policy from the Thai insurance group formed by the Thai government.
As father of Thai child...
If you are registered with the Amphur you can obtain a 60 day family visit extension for 1,900 Thai baht.
But your mother cannot get the added 60 days.
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