I'm not totally familiar with the earlier (pre-15) trucks, but how does the front struts in the 11 compare to the 04-08 F-150? I know the 15-17 Expedition front suspension is nearly identical to the 14 F-150, so was thinking that the 11 might be functionally similar to the 08 F-150, as the Expedition chassis tends to lag a generation behind the F-150.
The reason I bring it up is because as I understand it, factory rear air was an option, and Ride Tech sells their ShockWave replacement struts for the 04-08 F-150. They aren't exactly cheap, but combine those with the factory rear air shocks, an air compressor and tank, and a simple controller. Then you could possibly have the ability to lower the truck all the way down to the bumpstops to make it as easy as it can be in an Expedition for your daughter to get in and out, then raise it back up for normal driving.
Alternatively, while I can't find an off-the-shelf solution for the Expedition, there are several companies out there that produce custom coil springs and you could have a set made. Give "custom coil springs" a quick search and you'll find them.
