I assume your grandson would be playing on a "safe" computer. Is the minecraft server hosted locally or on a remote server? Will it run on a "safe computer"? Would it just work to put a temporary router between your main router and the Internet source?
What is the current topology of your network?
Computers -> Linksys -> Cable Modem?
I would just put an old router or computer set up as a router between your main one and the cable modem if that's an option:
Safe Computers -> Linksys -> Old Router -> Cable Modem
Unsafe Computers --WiFi/LAN----^
If this old router is a computer, you can even set up the minecraft server on it and then everybody, even the "Safe Computers" can access it. If not, you can port foward on the Linksys like in the tutorial:
I would also set the "Linksys" to be the DMZ for the "Old Router" just so that any UPnP ports from the safe computers still get back to them.
Or is it something like Computers -> FiOS Router/Modem?
This would be a harder topology to do, but you can still set up a three layer network like:
Safe Computers -> Linksys -> Old Router -> FiOS Router/Modem
Unsafe Computers --WiFi/LAN----^
All of this would probably be pretty plug and play as long as you set the IP address of the routers to different subnets. E.G.:
192.168.0.1
192.168.1.1
and
192.168.2.1