They are back rests... not seats. They aren't comfortable seats anyway, but the fittings that insert into the crossbars are not designed for the load of sitting on them. You won't hurt the boat because they are fully inserted into the front and rear crossbars. The bent tube structure inserts about.. 1 to 1 1/2 feet. Worst case is you fracture one of the fittings that allows the round tube to fit into the odd shapes of the crossbars. They are just poured urethane.
There is an obscure law of physics called the Law of Horizontal Surfaces. Meaning, if it can be flattened and spread out by kids, it will be. A subset of Murphy's Law.
