@JAF 

It honestly depends on what hardware version you have.  The PCs are different on the V2 and V3.  The V3 are more simplfied and will block website based on keyword matches an inherent black list.  On the V2 you have to create a whitelist (allowed) or a blacklist (blocked) of keywords.  The V3 allows you to set Bedtime ( when all access is cut off to selected devices) and access time (how many hours they can use the internet per day)  The V2 simply let you set hours they have access available. 

If specific sites are being blocked that are need likely you do not have their keywords in the Whitelist.  Whitelist again are sites that are allowed.  Everything else is blocked.  If your allowed list is smaller than your block list it makes sense to use a whitelist if its the other way around we would suggest using the blacklist.  

Best Regards