This thread is 2 years old, and the OP never came back... If you have questions on how to best leverage pfsense in your environment I suggest you start your own thread detailing your network and any questions you have on how to best do some specific sort of thing your wanting to accomplish.
To your question of squid, squid is a proxy package that can be used to filter access based upon a url that someone might access.. Like blocking access to www.facebook.com or only allowing access to say www.kidsafedomain.tld
But to be honest, some of these sorts of features are more advanced than many users (without networking experience) understand and would come with a steep learning curve if not already up to speed. Asking what squid is - points to not having the basic skilsets that would make deployment of such features an easy solution.. You might be better suited with a more home "user" sort of device - there are many "home" friendly devices with interfaces designed for point and click control of what kids can access..
Maybe something of such a list of devices will be of help
https://www.fatherly.com/gear/best-parental-control-devices-routers/