Correction first: SETV (Single-Entry Tourist visa) is valid for 90 days, as in you can enter the country within those 90 days. You don't get 90 days and extend for another 90. You only get 60 days, and you can extend for another 30 days at immigration for 1,900฿.
Your parents could apply for the METV (Multi-Entry Tourist visa), which if timed right, could give them 9 months in total. Although, they would still need to meet the financial requirements.
(Keep in mind that, depending on your parent's health, no matter what visa they get, if they want to stay 4-5 months, they will still have to leave the country to do a border run, which can be physically demanding and complicated.)
A good solution would be to get a SETV in Canada, extend it in Thailand (3 months total), leave the country and come back in on a visa-exemption stamp and extend it (45+30 days). That gives them over 5 months.
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