The best option is to put the computer in the living room or some open space, and asking them what do they do on Roblox. Parental controls is never an option to monitor your kids. If you don't want your kids to spend your credit card, tell to stop, punish, refund, or cancel your credit card. Also, if you are uneasy with roblox, it's best not to let your kids play it.

Ask your kids for their account password. Or put up some cameras around that computer.

To be honest, part of the spending problem is your fault. If you want your kids to stop spending with your cards, you need to try changing the device's settings to not allow purchases without specific approval.
If your kids easily just grab your cards, I suggest hiding them somewhere.
You can always make them play when you're around so that they're not unmonitored
That's all I can say about spending

PS, If you don't want your kids talking to strangers on Roblox, there's a setting that allows you to hide the chat bar. Look it up if you need a tutorial on how to use it.

Here's one thing you could set up a camera isolate them for a little bit leave your wallet on the counter see what they do you can basically monitor them with a hidden camera