How to charge iPad with one or more USB devices connected?
Use the Apple Lightning to USB Camera Adapter:

https://www.amazon.com/gp/product/B01F7KJDIM/

It has a USB port and a port for a lightning cable.  Just connect a powered or unpowered hub to the USB A female and attach your instruments to the hub then connect the new Apple powered USB camera kit with everything hooked up, plus the lightning cable to a computer or USB charger.