What is the total displacement of a child who walks 4 m south , 2 m north ,5 m south , 5 m north

The total displacement is given by the distance between the initial and the final points

To calculate the displacement easily, you can use and graph sheet of a plain sheet to draw each movement ( figure attached)

Here I have used different colors to signify different displacement

When we are done with marking all the displacement ,. we can find the distance between the initial and the final positions

Here it is a point which is 2 m South of our initial point and hence it is the total displacement