well there is an online version that should work through even an ipad browser. No idea if there are any versions for apple os. I've got 2 for android but only one is of any real use, but like I said it doesn't really work well with the onscreen keyboard. And balancing the tablet and wireless keyboard on my lap is awkward.
And if I'm going to sit down and do it - might as well sit down at a proper computer.

There must be drag and drop apps for ipads, yeah it's apple so you'll probably have to pay something for them (yet another advantage of android ) - but must exist.

Read this - http://forum.openscad.org/OpenSCAD-for-iOS-td21686.html

This is your best bet: https://openjscad.org/

But I don't really think it's that suitable for ipads.
This looks promising: https://www.gravitysketch.com/app/
Have to see if there's an android version.