Looks to me like its probably 2" square 1/8" wall (11 gauge) tubing. The next thinner 2" square is 14 gauge and it would be pretty flexible with two larger riders on that frame. Around here (at least a few years back) the 11 gauge was cheaper then the 14 gauge was, because the steel supplier bought more 11 gauge then 14 gauge. With the amount of tubing used in that frame, the difference in the weight between the 14 g and the 11 gauge frame won't amount to enough to warrant the extra cost and loss of strength of using thinner tubing., especially if you are considering motorizing the cart.
The tubing holding the steering column is probably 1 1/2" square.
I built a few adult pedal car frames, but around here 4 wheels required licensing, but a 3 wheels on the same size cart did not. The government in action!