They broke a rule and got a consequence. NTA. Actions have consequences. They were late.
He’s also projecting his thoughts about being a bad guardian about himself. Clearly have more sense than him by acting rational and not just driving off with loose kids in the car as he apparently would have. He’s projecting again about needing to lighten up as that’s what he needs to do if this upset him so badly. Someone keeping your kids safe deserves a thank you and not a chewing out.
sad but true