Pennsylvania mandates that all school sports team have a concussion return-to-play protocol in play. In hockey, most head injuries occur during group practice or games, so there is more than one official or coach or trainer witnessing the injury. If it happens during a game, kids cannot return without a doctor's note. For school sports, it's usually not the coach's call; there are school medical personnel who must receive the doctor's written ok. There are penalties for allowing kids back to play without a doctor's note, up to and including suspension and civil liability.
For club sports, the level of sanction depends on the club. My son was out of a game after a head hit. We brought him to the emergency room, and after lengthy conversations, it was determined he most likely didn't have a concussion. We still needed to watch him carefully for signs of brain bleeds; we could have opted for a CT scan, but there is evidence that in children 12 and under, CT scans on the brain raise their risk for later brain cancers. The doctor said he could play in 48 hours as long as other symptoms didn't arise. We waited a week. Even without a concussion, he was not allowed back to play without a note.
I'm sure there are coaches and parents who put kids back on the ice sooner or without a note. However, a young hockey player in our area just died from a brain bleed after a routine practice -- and I just found out that another young hockey player broke her back in a recent game after an illegal hit -- and I'm guessing there are civil suits to follow. So, at least in our area, people are a bit on guard.