I am a great fan of the behringer U-PHORIA interfaces, they do models from about £30 up depending on how many ports you want, and they are class compliant USB, so they don't need drivers or anything, as long as you have a USB for your iPad / phone (which are cheap to get, like the apple one). Any specific interface for the phone relies on them supporting it, like the line 6. Incidently the sonic port does still work on the previous OS, so if you get an older ipad that is not upgraded, it is still fine.
I have two iPads, my normal one and an older air that I gig with. My normal one (an iPad Pro 10.5) is up to date and the sonic port doesn't work, but my gigging one I don't upgrade, and the sonic port works fine on that.