CCMenu child of child selector not firing

I have a ccmenu with a ccmenuitemsprite in a ccmenuitemsprite which has the selector. I wish for the child of child to be a play button that fires a selector but its not firing.

CCMenu *menu [CCMenu menuWithItems: nil];
CCMenuItem *bg = [CCMenuItemSprite itemFromNormalSprite:[CCSprite spriteWithFile:@"sprite_sheet.png" rect:CGRectMake(0,50,100,100)] 
                                         selectedSprite:nil 
                                         disabledSprite:nil 
                                                 target:self 
                                               selector:nil];
CCMenuItem *playBtn = [CCMenuItemSprite itemFromNormalSprite:[CCSprite spriteWithFile:@"sprite_sheet.png" rect:CGRectMake(0,0,50,50)] 
                                              selectedSprite:[CCSprite spriteWithFile:@"sprite_sheet.png" rect:CGRectMake(50,0,50,50)] 
                                              disabledSprite:[CCSprite spriteWithFile:@"sprite_sheet.png" rect:CGRectMake(100,0,50,50)] 
                                                      target:self 
                                                    selector:@selector(onPlay:)];
[bg addChild:playBtn];
[menu addChild:bg];
[self addChild:menu];

If you want to pass the name of your images as arguments, please use the following code:

CCMenuItem *playBtn = [CCMenuItemImage itemFromNormalImage:@"image.png"
                                             selectedImage:@"image_selected.png"
                                             disabledImage:@"image_disabled.png"
                                                    target:self
                                                  selector:@selector(onPlay:)];

The constructor you are using is expecting a CCSprite, not an NSString.

OK, after you edited the answer, I looked more closely, and you have the playBtn as a child of bg. You can't do that. Only the direct children of CCMenu are able to send callbacks:

[menu addChild:playBtn];

I would suggest you do this actually:

CCMenuItem *playBtn = [CCMenuItemSprite itemFromNormalSprite:[CCSprite spriteWithFile:@"sprite_sheet.png" rect:CGRectMake(0,0,50,50)] 
                                              selectedSprite:[CCSprite spriteWithFile:@"sprite_sheet.png" rect:CGRectMake(50,0,50,50)] 
                                              disabledSprite:[CCSprite spriteWithFile:@"sprite_sheet.png" rect:CGRectMake(100,0,50,50)] 
                                                      target:self 
                                                    selector:@selector(onPlay:)];

CCMenu* menu = [CCMenu menuWithItems:playBtn, nil];

CCSprite* bg = [CCSprite spriteWithFile:@"sprite_sheet.png" rect:CGRectMake(0,50,100,100)];

[self addChild:bg];
[self addChild:menu];