After discussions with my sister, who happens to be in the pharmaceutical field and has both experience and access to information on many drugs, she has confirmed that the steroid only affects your teeth if it comes into direct contact ie as a liquid or aerosol. If the steroids are in either of these forms, you just brush your teeth or gargle with some mouthwash. The least you need to do is rinse out your mouth.
By crushing your tablets into juice or any other food, you increase the risk of direct contact as small fragments can coat your teeth. Swallowing the tablets whole means no chance of contact with your teeth. James has 3 tablets together, whole, with one teaspoon of yoghurt. Having it with yoghurt means that he doesn't need to chew, he just swallows.
Most of the time, the boys will be taking their meds in the morning, so this can easily be followed by teeth cleaning.
Sorry, hope I haven't rambled too much...