I’d say this is not so different from anything else your child might be studying. When the going gets tough, kids complain! Parental support and encouragement are really important, especially in something like music that takes a lot of time and effort and repeated practice.
Parents should work closely with the teacher to see if there are any issues and what solutions can be worked out, and talk to the child to find out what exactly might be the issue.
Sometimes the child simply isn’t interested in music, or sometimes the teacher isn’t suitable, and if it really is those cases, then it might be time for a change. But 95% of the time though, it’s a child being a child and there is nothing that an encouraging word and a hug would not solve!