The 28-year-old, who qualified on Monday after battling through as one of two fastest losers in the three semi-finals, produced a credible performance in a high-quality field.
She ran a time of 54.78, shaving 0.02 seconds off her semi-final time, in a race that was won by the defending champion and favourite Zuzana Hejnova of the Czech Republic.
Child, the current European champion, was Great Britain's sole representative in the final after team-mate Meghan Beesley failed to qualify from her semi-final on Monday.
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