Translate those mouthpiece skills directly to the instrument. Often kids can do it on the mouthpiece but they're using airstream or pressure - watch for this and correct as needed. High/low are NOT just a function of more/less air, especially at this level.
As for not practicing, there will always be reasons. Here are some tips I've found that are helpful:
Practice inside their parent's (or their own) walk-in closet. It absorbs the sound (all the hanging clothes!), usually has a light directly overhead, and is noise-isolated from the rest of the house.
Practice outside in a common area, if in an apartment. Most complexes don't have "quiet hours" until 9 PM. They can go outside and honk/buzz away for about 10-20 mins after they get home from school with no problems.
If it's a time issue, as in they don't have time because they are the child-care provider, encourage them to get to school early and be willing to stay late. Even if they can come see you and practice one day a week for ten to fifteen minutes, you'll make headway. Ask if it would be possible for them to practice when the little ones are sleeping, if they have that noise-isolated room. It may not be - but look outside the box and inside a different box for solutions.
In short, if they aren't getting "step two" or "step three" then go back to "step one" until they get it. We've all had those classes that just move SLOOOOOOOOW. Make sure they have fundamentally correct ideas in their head and produce correct sounds BEFORE you move on or you're just compounding worse after bad.