Is a 35mm steel square tube frame strong enough to support 100kg without flexing?
My structural tables don’t go “down” that low, but assuming A36 grade steel, (which is the most common in the U.S.), a 2 1/2” tube with a 1/4” wall thickness will support about 1,000 lbs. on a 10 foot span. Therefore, I’d assume two 35 mm tubes with 2 mm thick walls will “support” 100 kg without failing. However, I see several issues, 1) it will have significant deflection (flex, as you call it), 2) the legs are too small to keep the top from rotating, 3) 50 kg in the center of the top will depend more on the top than the frame, and 4) the joints act as “pin” connections without diagonal braces, so they’ll easily rotate. 

BTW, are you sure the floor can support that much weight?