You ask a very good question. I've run into this only about 3 times previously.
Two of the three times, what we did seemed, per the patient and parent to work. The third time we used a more radical approach, which had a limited positive effect only.
Here is what we did: There is more than two brands, but the only two that comes to mind is "Mavala Stop" & "Stop it". Can get them at Walmart, Amazon, etc. It can be painted on the outside portion of the night guard and inside portion of the lips at night & tastes terrible!!! It HOPEFULLY would assist do the trick, or at least assist some minimally.
The worst case scenario, assuming the above method did not work sufficiently is: To have your Dentist or Oral Surgeon, have the teeth wired closed at night. This is much more involved. It also brings up problems with brushing/oral hygiene, and thus would require you having to have the wiring taken off and put back on periodically with brushing in between the rewiring. I do NOT recommend this latter choice - but it has been employed - at least for more involved and severe cases.
Use your judgement, but the easy & first choice, at least in my estimate, would be to try the bad tasing material for a couple weeks or so, or until the habit is broken...! I hope this this helps you. You sound like a very good Mom! Dr. Rampton