Apple makes a Lightning to SD reader; I presume they have or will have one for the new USB C iPads that would work directly with your SD cards.
If using a different card reader, I believe it will depend on the current draw needed by the card reader. On my previous generation iPad Pro, I need to use the USB 3 camera kit cable while plugged into a power source that can deliver more than 5W (depending on iPad charge level) in order to use my USB 3 Compact Flash card reader.
The new iPad Pro models may be able to provide more power to USB devices. I think I read the spec somewhere but don’t recall which review had it.
If connecting the camera directly to the iPad, you should be good without an external power source because the camera can provide its own power during the transfer.