That's tricky. In my experience with families, the child or youth and their parents are very often of the same mind. I've had a few encounters in which parents and their child had different opinions about what they wanted for treatment, but as the child's illness progressed, I can't think of a situation in which the child wanted one thing and the parents wanted something different.
With MAID in general, there are situations, sadly, in which the person who opts for MAID does not have the support or approval of their family. That may actually play out in the world of children and youth, but for the most part there will be concordance, and for the rare cases in which there isn't, I think that going with the decision of the mature minor, with all the caveats that Dr. Zlotnik Shaul just laid out, would be acceptable.