Looks neat:
http://www.roblox.com/I'm intrigued by the use of Lua as scripting base. Kid friendly is a change of pace; the massive amounts porn in Second Life gets old quick.
Here's what's happening, your network usage is through the roof when the client for this is used. That's it, nothing more. Anyone who's played online games has experienced one or two bandwidth hogs. Ideally the software itself will offer ways to alleviate your pain. From what I can see, this one doesn't:
http://wiki.roblox.com/index.php?title=Hardware#The_InternetIf your router has QoS ( Quality of Service ) you may be able to throttle the one machine's connection down so everyone else can play too. Give it's Windows only, I'm not in a position to test it. There may be some Windows client solutions that can impose a hard cap on transfer.
You router could also be fighting it. Standard firewall operations can cause programs to try over and over again when something is blocked. Some info here:
http://wiki.roblox.com/index.php?title=Fir...d_router_issuesGood luck.