Not happy indeed. I hope you didn't get hit with too many $10 excess fee.
Not sure how old they are but I generally start kids on prepaid plans - that way they can never go over. If they have Android devices you can also set a monthly warning and limit so that they can be allocated a set amount (which turns off once they reach it)
You can request Optus shut off data (and then turn it back on) but you have to call and its clunky. At this stage I would just tell them they have to turn off mobile data. They can still use wifi obviously (unless you change the wifi password). Install the Optus App on each phone. You can see the daily data usage in the app and so just check its still 0Gb in a few days (loss of the phone if it isn't). Or you can check in with Optus to confirm no data usage on their numbers.
Better IMO to get them knowing how to manage their impulse to consume data rather than do it for them.