Staff at Banksia Hill Detention Centre refused to unlock inmates' cells for about 50 hours after the violent escape of two prisoners last week.
Community and Public Sector Union-Civil Service Association secretary Toni Walkington said the lockdown was enforced while negotiations were continuing over security concerns.
"Our members refused to unlock the children from their cells until demands for improved safety were addressed," she said.
"It was agreed on Saturday that emergency support group personnel from Corrective Services would provide security assistance and escort construction workers to and from the site."
Two teenagers escaped from the Canning Vale juvenile detention centre on Thursday after assaulting a construction worker, stealing a four-wheel-drive and crashing it through three security doors.
One was caught at the weekend by police called to a wild party in Nollamara but the other is at large.
In Parliament yesterday, Corrective Services Minister Murray Cowper said extra security had been guaranteed, including a 3.6m fence around the construction site at Banksia Hill, while the centre's extensions were built.
"It's going to take about three weeks to put in place that additional security," Mr Cowper said.
Shadow corrective services minister Fran Logan asked the minister when he was made aware of concerns about security at the centre.
Banksia Hill staff rallied at Parliament in June and spoke to former minister Terry Redman.
Another concern was that an anti-ramming device at the entrance was not working when the escapees smashed through the last roller door.
"I've called for a full investigation into the circumstances," Mr Cowper said.
He said the number of escapes from WA prisons had declined since the Liberal-Nationals Government was formed in 2008.