Proxy, firewall filter with controls?

I have a network that is used by children, and adults. It is wireless and connects to about 10-12 people using a wireless/wired router connected to a switch. I want to set up a proxy server for several reasons such as filtering, caching and monitoring. I hope to find a good one that is simple enough for people to manage in my absence. Cheap/or freeware would be nice but I really dont want an enterprise level. thanks