The 26-year-old from Perth was more than half a second in front of the field as she finished in 54.42.
“The conditions were not perfect, so I couldn’t believe it was a PB [personal best],” she told BBC Sport.
“I just wanted to attack the race from the start because I had the wind behind me and keep the hurdling smooth.”
Child, who reached the semi-finals at last year’s Olympics, led from the start and eased to a dominant victory ahead of Hanna Yaroshchuk from Ukraine and Russia’s Vera Rudakova.
“The last few faces I’ve been finishing far too strong and I’ve not been going out hard enough,” she explained. “So, I think I got it just about spot on.”
The 2010 Commonwealth Games silver medallist had achieved new Scottish records on the flat over 400m indoors earlier this year. And Child’s latest impressive run puts a marker down for the World Championships in Moscow.
Also on Saturday, Mo Farah toyed with the field before unleashing a breathtaking final lap in the 5,000 metres race.
The double Olympic champion, competing in a British vest for the first time since his London 2012 heroics, just about jogged the first 4,600m before taking off, running the last lap in 50.89 seconds to crown a day on which Great Britain claimed five wins.
Farah said: “I had a text from (coach) Alberto (Salazar) this morning to just wait as late as possible.
“I wasn’t allowed to do anything, I wasn’t even allowed to go to the front, but I was tripping over as the race was so slow, so I went to the front, ran as easily as possible and went from the bell.”