How can I stop Roblox from re-downloading on kid's iPad?

You can block installation (and/or deletion) of all Apps, but not selectively via ScreenTime settings:

Settings > ScreenTime > [Restrictions] Content & Privacy Restrictions > [Store Purchases & Re-Downloads] >

• Installing Apps - set to Allow or Don't Allow as required
• Deleting Apps - set to Allow or Don't Allow as required

If you haven't done so already, you should set a ScreenTime Passcode that is known only to you:

Settings > ScreenTime >

When setting a ScreenTime Passcode, you have the option to associate an Apple Account that can be used to reset a forgotten ScreenTime Passcode. You would be wise to use your own Apple Account (AppleID) for this purpose - and not your child's own Apple Account.

After you have denied the ability to install or delete Apps, it follows that you will need to temporarily change the associated permission if you wish to subsequently add or remove Apps from the iPad - restoring the restriction when you have finished making any changes.

[Edited by Moderator]

What you are attempting to achieve isn't possible with the available controls.

Apple invite submission of feature requests via its Product Feedback portal. If you would like to see additional functionality or controls in a future version of iPadOS, this would be the most appropriate channel through which to make a request:

Feedback - iPad - Apple

It's either all or none, as posted already. These may also be relevant in this situation:

See these tips on possibly being able to prevent use of this app by a child:

How can I cancel previously approved apps… - Apple Community

You can use Screen time to block the (previously approved) app. Screen time.. App Limits.. Add Limit.. Then pick app.. Set time limit to 1 minute (not sure why 0 minutes, doesn't work, but it doesn't), then set to "Block at end of Limit".

Block a Child's App that's already been d… - Apple Community

The only work around I have found this far is to set the screen time limit on that app to 1 min, then they will have to request more time to continue using it. It’s not ideal but it seems to do the trick for now, unless you think they can get into trouble in 1 Min time.