Tem como a thread main saber quando todas as threads filhas terminaram sua execução?

Você pode verificar se a Thread está viva com o método Thread.isAlive()

https://docs.oracle.com/javase/6/docs/api/java/lang/Thread.html#isAlive()

public class JavaIsAliveExp extends Thread   
{  
    public void run()  
    {  
        try   
        {  
            Thread.sleep(300);  
            System.out.println("is run() method isAlive "+Thread.currentThread().isAlive());  
        }  
        catch (InterruptedException ie) {  
        }  
    }  
    public static void main(String[] args)  
    {  
        JavaIsAliveExp t1 = new JavaIsAliveExp();  
        System.out.println("before starting thread isAlive: "+t1.isAlive());  
        t1.start();  
        System.out.println("after starting thread isAlive: "+t1.isAlive());  
    }  
}