Using ipad as Internet connection in a wifi hub
Unfortunately none of Apple's routers can use the iPhone or iPad tethering as a source internet connection.

It looks like it might be possible using certain builds of the DD-WRT custom firmware with the Linksys WRT160NL, but based on that link, it looks like it will likely require a fair bit of hacking, beyond just installing the custom firmware (i.e. it needs to be certain versions, and it looks like further drivers need to be compiled/installed).

If you have a Mac handy, you could tether the iPad to it (with USB or Bluetooth), then use the Mac's Internet Sharing to share to the other devices over Wi-Fi.

Alternatively, you could look into getting a 3G/LTE hotspot device from your wireless provider. It would likely entail some additional cost, but it would be most suited to what you're wanting to do. Some providers offer some data plan sharing amongst devices as well, so the additional costs might be minimal.