how to backup iPad with Apple Airport Time Capsule

Not possible to do this directly. See Apple's support document in this regard:

https://support.apple.com/en-us/HT203977

There are two ways to backup the iPad:

1) iCloud

2) iTunes on your Mac

IF.....you back up to iTunes on your Mac......AND......you have the Mac setup to backup using Time Machine, then the iPad backups stored in iTunes on the Mac will be backed up to the Time Capsule.

So, the best that you can do is back up the iPads (and iPhones) indirectly to the Time Capsule.