I can see how this COULD be necessary. Kids do have moments when they'll kick, punch, spit, bite - themselves or others. A straight jacket would have been better, but do they make them so small?
Anyone who has seen a strong 4 year old completely melt down violently can tell you - they're tall/strong enough to break their own bones easily, and have zero idea of the consequences. If junior is throwing himself off the couch or punching glass windows, parents might have to literally hold them down and calm them. If it's a cop doing it, he could get sued for that, since everyone has a phone and everyone has a better idea what should be done.