Trying to balance bright daylight, needs a lot of power
A big reflector ( Sunbounce ), is good but cumbersome, big advantage, little to go wrong and you can "see" what you are doing. You need an assistant
Continuous lighting is unlikely to have enough power, unless the day light is poor
If you can dictate the time, then sunset is ideal
In the middle of the day, the only soft flash that is going to work is a powerful, expensive, studio unit
On camera direct flash (with out a diffuser) is the cheap and cheerful solution . Try doing some practice shots under different lighting using models of different ages
keep the ISO as low as possible and stop down to f8 if there is any depth to the group