A few general observations are called for now. Trying to get through to a child is one thing, but if the dentist himself/herself is the only person instilling good habits and protocols, the child will likely not stick with the good habits. It is imperative that the parent set the guidelines and it is equally important that the child sees the parent practicing what they preach. If the parent is simply giving lip service but is not doing what he/she tells the child to do, it will almost certainly not stick (at least not as well) as it would if the parent is doing what they are asking the child to do. This is not unlike most good habits.

The parents should also make it a point to show proper technique and monitor the child discreetly as they do their brushing and flossing to make sure that the child is doing it correctly and for the proper interval of time. The monitoring should not be over-bearing or micro-managing as the child will likely get frustrated. However, it must be done because it will set the stage for good (or bad) habits in the years to come for the child.

Avoiding sugary drinks has other benefits as well. People that do not drink many sugary drinks are generally healthier and slimmer. While...

As such, it is important to help the child avoid an over-consumption of sugary drinks both for dental reasons as well as general health reasons to avoid diabetes, obesity, and other health maladies to which over-consumption of sugary drinks can create or at least aggravate. Even a step down from soda to drinks like Gatorade can help as this alone usually cuts the calories in half. Any steady use of energy drinks like Red Bull should be avoided and this is for dental reasons as well as health reasons that far eclipse standard drinks.
Conclusion

At the end of the day, the child just had a cavity so the overall prognosis is certainly not bad. However, the child needs to gain the good habits of brushing and flossing lest they end up having to get a series of fillings, root canals or even extractions over time if the bad habits continue. Orthodontia may or may not be needed but that will become much clearer over the next five to ten years.