EDIT 06/20/2023
As of the same date of this edit, Roblox has officially moved to ban simulated gambling by September 18th, 2023 (in 3 months), which means that all forms of gambling are no longer allowed on the platform. This contrasts the prior stance to which, as long as there was no Robux involved, it could be passable.
The advice in this post is no longer accurate. Please avoid sharing this as an answer. For more information, please see Updating our policy on simulated gambling in the thread Create Experiences for People 17 and Older on Roblox.
You’re immediately in the clear when you meet the minimum requirements of having no Robux involved in the system directly or indirectly and if the chances are not skewed or rigged in favour of the developer in any way. I source this primarily from the Community Rules which leaves little room for doubt.
Gambling with Robux or real money.
— Section I Rule 11 (Dangerous, illegal and unethical activities).Do not implement any gambling or casino style games that involve real money in any way. This includes Robux, as Robux can be purchased for real money. Games should also be fair and not be skewed in the developer’s favor.
— Section II Rule 1 (Building your game).Roblox Community Rules. Accessed 20 November 2020.
Anything beyond that is completely your choice and simply adds contributes towards restrictions or anti-addiction strategies. Limiting the amount of times players can gamble per day or other time period is not a requirement, though it might be a comfort strategy so players aren’t constantly participating in gambling features in your game. It may also help into a one-size-fits-all solution for countries that don’t outlaw gambling but place restrictions on the activity.
There currently doesn’t exist a way to restrict features according to a player’s age and there’s no need to do that either. As far as money loss goes, this is a separate topic on its own. Roblox has two separate policies handling Robux purchases and items. If it’s possible to always win something from your activity, then you must start adhering to the random virtual items policy rather than the gambling policy.
The restrictions are a little more relaxed when it comes to paid random virtual items: you can involve Robux here, but you must then include true and full disclosure about the chances of earning rewards from your activity (either by rarity, by individual item or any other appropriate measure). There are some regions where you need to restrict use of these generators however; that is where you will need to involve PolicyService and read off ArePaidRandomItemsRestricted from a player’s policies.
Here are some resources around player policies and the random virtual items policy:
Gambling is available plain and simple in the Community Rules, there’s no dedicated thread.