I see it rather often. My sister and brother-in-law will do it when traveling with my nephew. If the seat remains empty then the kid gets somewhere to sit. If not, he stays on one of their laps. And they trade seats with the person assigned to the middle so they aren't passing the kid back and forth over the other passenger. And if the parents ask nicely at the gate they often find the seat blocked if the plane isn't full. It really is better for everyone on the plane if the kid isn't screaming and the extra seat helps.

It is not uncommon, not underhanded and hardly a shenanigan, IMO. If the kid was too old then it is another story, but that doesn't affect the seat assignment part of things.