-(void) setOpacity:(GLubyte) anOpacity
{
opacity_ = anOpacity;

// special opacity for premultiplied textures
if( opacityModifyRGB_ )
    [self setColor: colorUnmodified_];

    [self updateColor];

    for (id<CCRGBAProtocol> child in children ) {
        // You should check if child responds to selector or conforms to CCRGBAProtocol.
        [child setOpacity:opacity];
    }
}

- (void) setOpacity:(GLubyte)opacity
{
    [super setOpacity:opacity];
    for(id<CCRGBAProtocol> child in children) {
        [child setOpacity:opacity];
    }
}