I tried to post a comment to Quinn's question but the code formatting for comments is ugly. Here is how I'm starting the child app:
let process = Process()
let url = Bundle.main.bundleURL.path + "/Contents/MacOS/Child.app/Contents/MacOS/Child"
process.executableURL = URL(fileURLWithPath:url)
process.terminationHandler = { (process) in
print("\ndidFinish: \(!process.isRunning)")
}
do {
try process.run()
} catch let error as NSError {
NSLog("Failed to execute ", error)
}