The lighting is somewhat flat, but also inconsistently directional. The sun being on the right means she has some directional lighting, but he's in her shadow, lit mostly by direct on-axis flash. What bothers me the most is that because of her shadow on him, they appear somewhat lit from opposite sides, with shadows on the sides of their faces that are facing each other - that's incongruous.
A solution would have been to get the flash off camera, ideally in a softbox, placed camera-right at an angle to the subjects. That would have maintained directionality of light, but added fill from front-right. A second light at low power or a half-gold reflector at camera left could have been used to slightly fill in the shadows opposite the main light and sun.
And for a shot like this Nuero's advice is as much as you probably need, your clearly working with a pair of Professional Models.
Getting the Flash Off Camera is the first step, look for someone to Hold the Flash as Nuero suggested, off to the right (several companies produce Flash Brackets, I use the Ring Flash Brackets from RRS with an extension).
Check out Lastolite for soft boxes, and Westcott for Reflectors, you can get soft Boxes to fit just the flash head & Reflectors come in pretty well any size you want.
When setting up for portraits, always give a moment to look behind the Models, Background is So important, Look for a soft dreamy background (If you Can), that with say an f stop of 5.6 will render your Models in Focus, but drop your background out of focus, check for Trees growing out of your Models heads, bright & shinning patches of light reflected, Yellow Buses etc, try to keep the background where possible uniform so it doesn't distract from the main subject, your lovely children.