Here is Enya's reply:
At first,When he see the Barbies,he is so miss Mariam.Because Mariam has a Barbie.he feels worry about how is Mariam now.Maybe she is hungry now;Maybe she is sleeping on the street now;Maybe the Talibans are hurting her now……Then,he feels afraid with these Barbies."He could practically feel Mariam's tiny fingers slipping away from his as the phantom rumble of a truck echoed in his mind " [p161--162] He can see what happened again the day Mariam was lose in his mind. He is so ashamed for it was his fault that Mariam was lost ,and he never tell his family.So every one thinks it was their fault,but it is himself's.At last,he feels so angry.Because when they will get onto the truck,Mariam lose her favourate Barbie---Gulmina. So Mariam starts struggle in his hand and she is lose! If there isn't a Barbies, Mariam would live with them together now!So he starts banging the Barbies.