Since the mains are the strings that tend to shift the most especially with heavier topspin hitters you will find that if you go with the x-one biphase in mains that it will be constantly rubbing over the thin,hard 18G black code in the mains and thus cause earlier then desired breakage of the x-phase. Been there tried that and found that my 12 yo daughter would break it within a week. If you still want to try this set up at least go with 17G black code.
For the above reason I would suggest, as previously mentioned, to go with Multifilament(x-one biphase, 16G) in the crosses and the poly(black code, 17G) in the mains. Now the poly moves across the x-phase and will tend to last a bit longer vs if arranged the other way around. With arrangement it lasts my daughter 2-2.5 wks before breakage of x-phase. Go with this set up for greater durability, potentially more spin generation and still very easy on the arm for a young player.