FWIW Child Safe of recent seems to lack stability with posts on the forum noting:
- changes not being saved
- saved changes reverting without obvious cause
Should you still wish to use Child Safe then sign in here https://www.virginmedia.com and:
- select Account settings > Online security > Websites
- type roblox.com under Blocked Websites section and then select Add
- select Save changes and confirm changes are saved as highlighted below; should changes not be saved try repeating previous actions:

Do keep in mind that Child Safe and similar services (like Setup Guide | OpenDNS) are not substitutes to age appropriate supervision.