OP:
For the use case you described above, I would NOT get them either an iPad or a 21" iMac.
Instead, I'd get them a 27" iMac with the bigger display.
I'd get 8gb of RAM (can be added easily if you wish, adding RAM IS NOT possible on the 21")
I'd get an SSD inside (either 512gb or 256gb).
I'd get the basic or midrange CPU (they won't need "upper end").
You want A BIGGER DISPLAY because their eyesight may be "getting older" and they'll appreciate the ability to "enlarge text" if they need to, or choose a lower resolution as their "default resolution".
You may not be concerned about this, because you're younger.
But THEY will understand.