Maybe I’m being a bit thick here, but I don’t get what you’re trying to acheive…
If you have a wireless router connected to Virgin, via a cable modem, then all you should have to do is connect the 360’s with the supplied ethernet cables to the router, and then connect the laptops wirelessly to the same router. Assuming the router is good enough to sort out all those connections, I don’t see why you need to be faffing about trying to connect through the laptops… Maybe I’m missing something!
However you internet connection is a bit slow (bandwidth) and may make online gaming painful
Depends whether both sons are trying to get online at the same time. My Virgin connection is 2MB (same as OP) and have never had any lag issues when playing on xbox live, but if you have another user downloading/using the net on a seperate device then that might give you some problems.