Despite "emission" being in the name of the ULEZ, I don't think the article is necessarily implying that the mechanism is specifically that kids were more willi

Despite "emission" being in the name of the ULEZ, I don't think the article is necessarily implying that the mechanism is specifically that kids were more willing to go outside because there was less pollution. There are various other possible mechanisms: parents decided not to drive their kids based on the fees, there were fewer cars on the road which made it more pleasant for kids to walk to school, etc.