How to allow server access but restrict internet access?

Our Vista laptops (& my Xbox) access our basic Windows Home Server system
through a Netgear router. It's all fine, but I want to (easily & quickly!)
restrict & control when some laptops can access the internet while allowing
free access to the server. (For instance, so children's internet access can
be monitored but they can always get to photos & backups, etc. on the
server.)

What might be the best way to do this?
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