Wifi, without an access point.

Alternatively, you could use an AP of your own, with no internet connection. (This is the easiest way, if equipment is not a problem.)

If you really want a wired connection, your only option is USB; you can do USB->laptop networking, but i've never got that to work with Windows. You could do USB->ethernet, which I've done, and then use an ethernet switch to hook up the tablets and laptop.

But it's easiest to just set up a WiFi network.