The child really wants to learn but the parent isn't interested and employs tactics such as forgetting the instrument, keeps them busy all the time so there's no time to practice and has arranged lessons on an alternative instrument even though the child has expressed a preference for the violin and is good at it, having a natural bow hold and good coordination between bow and finger placement etc
Is the parent actively discouraging their child? Or is the parent simply a busy person who doesn't care one way or another about whether their child plays violin or does any of a dozen other activities? Because they're presumably paying for lessons, which implies a level of active support. And is the child actually both invested in playing and unable to remember their own instrument? Because "do you have your instrument for lessons" is a reasonable level of responsibility to expect of an interested child. And all those other activities. Is that parent insisting or child expressing interest and parent indulging?
No matter what the situation is, you won't know what's going on until you actually talk to both the parent and the child about your actual concerns.
If the parent is truly discouraging, you won't get past that. If they're simply uninvested, find out why and ask what it would take for them to be willing to invest more. Also, put a basic level of responsibility on the child. They should be expected to remember their own instrument at the least.
Forgetting the instrument
If you have an instrument the student can use during lessons, then that's probably your best solution. But understanding this may not be possible, then your next option is to practice things that don't require the instrument: music theory (at a keyboard if available), sight singing, listening (to expand repertoire and general musical awareness and interest), or any other activities you can create.
No time to practice
One must always teach the student according to what they're ready for when the arrive at the lesson, whether after a good or bad week of practice, or after a lot or a little or no practice at all. Your lessons guarantee that the student is learning at least as often as you meet.
The parent is the parent, and the music teacher is the music teacher. It's the teacher's responsibility to provide the best learning experience possible — during lesson time. The practical problems of "forgetting" an instrument or not having time to practice can be adapted to. The parent-child relationship is outside the teacher's purview. Consider that any attempt to address your feelings with the parent's choices could well result in losing the student, in which case they cannot benefit from what you are able to provide within the current limitations.
Obviously not an ideal learning situation, but how often is the learning situation actually ideal? None of us know which interested or uninterested person will prevail in the end, but as long as the interested person receives support and validation of their interest from others, chances are better that they will make progress in pursuit of their interest. Uninterest on a parents part only increases the responsibilities that the student has to accept in order to continue to pursue those interests. If the student understands their own increased responsibility, they very often will step up and accept that responsibility in order to keep going after that interest. If they can do that one action, accepting responsibility for their own success, it will be much more difficult to impede their growth and development. Of course success is never actually assured even if the situation is ideal. Such is life.