The 28-year-old Commonwealth Games silver medalist and current European champion finished third in her heat but will run in Wednesday's final as one of the two quickest losers.
The top two in each of the three heats qualified automatically but Child, who competed in the first semi-final, finished behind the USA's Cassandra Tate and South Africa's Wenda Nel.
But her time of 54.80 seconds was enough to secure one of the final two slots and she will be Great Britain's sole representative after team-mate Meghan Beesley failed to qualify.
The final will take place at 1.10pm UK time on Wednesday.
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