It's under exposed for starters, probably because the meter was fooled by the back/side-lighting. Common problem - dial in +1.0 or so exposure compensation.
But the shade side will still be darker, and fill-in flash is the easiest way to balance that, though a reflector if you have one is just as good, if not better. Adds a nice sparkle to the eyes as well. The pop-up is actually very good for fill-in. Pretty much just switch it on and shoot, then adjust the balance of flash and ambient if necessary with the independent compensation controls in Av (that's one easy way, there are other ways of doing it).
There's a danger that in bright sun the shutter speed will naturally rise above the x-sync speed, so you need to raise the f/number (or reduce ISO) to counteract that. This is what high speed sync is for, but no pop-up has that feature.
BTW, shooting into the light like that, then filling in with flash, produces great results - sun in the hair, no squinting into the sun etc.