The former couple divorced in 2009, with the Voice coach receiving primary custody of their two sons in 2012.
In the court filing, Foster's attorney, Angela Kinley, claims that "the children are at risk while in the care of third-party caregivers...while (Usher) travels around the world for his work-related activities."
Foster is asking a Georgia judge to immediately award her temporary custody of all her minor children currently under Usher's care.
On Monday, 5-year-old Usher Raymond V was hospitalized after almost drowning in his father's pool while trying to retrieve a toy that was stuck in an underwater pool drain, according to an Atlanta police report.
Usher Raymond V's arm got stuck in the drain while he was underwater. His aunt and a housekeeper unsuccessfully tried to free him.
Two male contractors working inside the house then rushed to the pool and were able to free the boy. One of the men performed CPR, reviving the boy before medical responders arrived at Usher's home.
As for the emergency hearing, it is scheduled for Friday at the Fulton County courthouse.
—Reporting by Claudia Rosenbaum