HI Kiki,
there are few solutions to your problem from simple to complex (and versality).
Let me illustrate you with few concepts as startpoint and make them available for other people with the same question (or related).
FIrst of all you should know about lighting adapters which make iOS devices cappable of so many different things from hdmi video output to camera connection kits. with the release of the new iPad pro Apple has released the new "Lighting to USB 3" adapter which permits plug usb devices too (and usb hubs 😉). This adapter is amazing due it finally charges the iOS device in addition to give the connectivity (which wasn't possible with the old cck) making the route "adapter+hub+soundcard/controllers) the most versatile.
Can I plug any usb device through this way? Well only if these are so called "class compliant" (driverless) ones which are so many like focusritte scarlett series, lexicon omega, some new behringer u-phoria... Olds news and newcomming. Also midi controllers such korg nano 2nd gen, most novation ones (launchpad mk2, mini, pro, lanchkeys, launch control...) and so many more from roland/edirol, m-audio etc. This last group includes midi interfaces like midisport 1x1, 2x2, 3x3... (Class compliant everytime remember).
So if you connect to the lighting USB 3 (new kind of CCK) a midi interface like those 1x1 you could plug your midi stuff to it and go! Also if you have a soundcard with midi interface and this is class compliant it will give you the same possibilities plus audio cappabilities from it. Record, playing, headphones... It isn't wonderful?
Also you can find dedicated solutions like Alesis IO dock (2), Focusrite iTrack dock and even old 30 pins docks (with the proper ligtning to 30 pins adapter of course) such Alesis io dock 1 or behringer is202. If you search at Auria forum you will find lots of resources from users testing and sharing tips about all those interfaces and gear.
I hope it helps you, mate!
R'n'r!