You asked about the letter from your pension plan before and the answer hasn't changed. Thailand doesn't recognize that letter. Only income certified by Embassy or 800k baht in the bank. Or an Agent.
The only suggestion I can give you is for Visa information go to the Thai Embassy website and look at the various Visa options. Also international schools are very expensive here especially with the number of children involved. Sorry I cannot give you more info. Someone with more knowledge will help you out.