I was in the same situation. I was promised it would all work out, yadda, yadda, yadda. (I didn't want FM modulated and I didn't want to buy multiple ipads, keep them charged and keep track of them. Plus, I would have very little control over what the kids were doing on them while we were driving. Additionally, if they had a problem, I couldn't help. )
I emailed Voxx and here was their reply (11/30/17):
The system that is being offered and installed in the 2018 Ford Expeditions will only be available as a factory installed option, there unfortunately will not be a version available at this time for after sales.
The Entertainment System does come equipped with 2 wireless headphones for listening so that others in the vehicle can still listen to the radio station of their choice.
We decided to purchase one with the factory installed headrest. I know it will work and I can control it through from the nav/screen. "You're fighting? No movie." (Yeah, I'm mean that way.)