The easiest way would be to do a quick fade to white instead of black by placing a white solid color on the lower track. To make the flash more like a flash bulb going off, you might want to do a cut to white with a few frame dissolve to the next image.
I did almost what John suggested, a few years back, only Instead of putting white below (which seems easier ;?), I think I keyframed the event ends to white. The results were stunning!
If this is an attempt to imitate a Flash Bulb, then experiment with the shape of the transition curves. IMO you'd want a curve that weights/places the POP at the beginning and then a gradual fade away. However, after a few reptitions this could get truly annoying.
Another "gimmick" to add would be an ever so slightly zoom in or out with a target graphic just before or over the click. You could also add motor and shutter/click Foley.
Cute kids. I can see a bit of interlace comb though. Stills are captured through the video preview window. When you capture a still you should temporarily set the preview settings to highest quality and so that the preview is deinterlacing the image. There used to be a free script floating around that would do all of this for you.