Im going to assume the child is still a Thai citizen, and has a Thai passport. If you are wanting the Non-o visa based on supporting a Thai child, it will cost 2K THB for a single entry and 5K for a multiple entry visa.
The specific information can be found at
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บริการสำหรับคนไทย/NON-IMMIGRANT%20VISA%20%28O%29%20FOR%20DEPENDANT.pdf . Make sure to print it out and put the documents in the order requested