There will be better, more factual answers. But when I was about 10 we had a science show at school where they lit two balloons on fire - one with air or helium

There will be better, more factual answers. But when I was about 10 we had a science show at school where they lit two balloons on fire - one with air or helium and one with hydrogen. They were about .7 meters in diameter. The second time the show came to our school, I knew what was coming with the hydrogen balloon, and, hating sudden loud noises, excused myself to the bathroom until it was over. I think I stayed there about 15 minutes before they had to come check on me. So at least 15 10-year-old-bathroom-minutes explosive. In short, very loud. But if they do it repeatedly in front of a hundred children, probably not too much. I don't remember feeling any pressure wave. But I'm guessing that a hydrogen leak in an enclosed space could easily result in a dangerous H2/O2 mixture.