To my knowledge, an app has to request for permission to send notifications before the app shows up in the notification settings (correct me if I am wrong).
To have Roblox request for permission to send notifications, open settings (in Roblox) and choose notifications. A green button should appear that says enable notifications. Then, a pop-up should appear that says "'Roblox' would like to Send You Notifications"
If that does not work, open chat for the first time on Roblox. 2 separate pop-ups should appear asking if you would like to enable notifications both in-app and on your iPad. Hopefully one of these works.