Floors are dirty - whether they are airplanes, airports, playgrounds, or any other place a child plonks itself. I think most past generations survived, I don't recall any parents telling us off for sitting on the ground places - in fact, it was the norm in a lot of places (school assemblies for example). Those floors were shockingly filthy at times (one hall had a serious air circulation problem and we would have grey butts when we stood up from the dust on the floor!)