I've talked about this a bit over in the Studio Workshop forum.
By far the simplest solution is a miniplug to dual XLR or 1/4" TS cable from your iPad's headphone jack to your mixer. The converters in Apple devices actually sound pretty darn good, and I use this method of getting audio out of one of my iPads all the time for live shows on the radio from my studio.
If you want to get a little fancier without spending a ton of money, I recommend that you get an Apple Lightning To USB3 Camera Adapter
. Anyone and everyone who wants to be serious about iOS-based music should have one of these; at $39.99 or whatever, it's a stupid cheap solution to a lot of problems.
Basically here's how it works: you plug the Lightning jack into your iPad. The other end has two ports. The Lightning port is a passthrough for power; you run your standard Lightning docking cable from that port to any USB-based 5V power supply, like an iPad charger. (Or your Mac.)
The other port is a USB-A port, and you can plug literally any class-compliant audio or MIDI device in the world into it and it will work. iOS uses the same Core Audio and Core MIDI drivers as macOS, so there's no need for special drivers or anything; it just works. I have done this with everything from a 2-channel audio interface to a 32-channel digital mixer, and have never had a failure.
That little dongle lets you do so much with your iPad or iPhone it's not funny. Right now in my studio, I have one iPad plugged into a Novation AudioHub 2x4 which passes MIDI from a Linnstrument from its USB hub, another one plugged into an Alesis Control Hub for audio output and 5-pin MIDI I/O, and another one plugged into a Yamaha AG06 mixer. Note that some of these devices require their own power and will still need to be plugged in, but even then, the Lightning cable will keep your iPad charged throughout your show so you don't have to keep glancing at the battery indicator.
There ARE dongles that have MIDI In or I/O, audio out or I/O, and charging ports (two examples are the IK Multimedia iRig Pro I/O and the Korg plugKEY); those are fine for music use, but the Lightning to USB3 adaptor does way more, from accessing digital cameras to external storage devices like SSDs and USB thumb drives. I have about five of them around my studio and in my backpack.
Oh, and for those of you who think Apple's prices are a ripoff and you can do just as well with a $10 knockoff adapter... For these adapters to continue to work smoothly as iOS advances, there are occasional firmware upgrades to the tiny chip inside the adapter. Once in a while you'll be told that your adapter is being updated, and five seconds later, it's good to go. Imitation adapters don't have this firmware, or it's burned at the factory for a particular version of iOS, and the odds are good that eventually the adapter will simply stop working and require you to buy another one. And another. And another...
I do use headphone cables with my iOS devices that have headphone jacks, but it's so easy to hook up real interfaces and get more robust cabling and better sound, why not?