I'm going to reframe the question from an unschooling perspective: how do your kids feel about "being coached"? Is it something that gets them inspired and excited about what they're doing? Does it make hard work into a slog? Does it set them up to weigh and measure their own worth as human beings against a set of external standards? Does it build them up, give them a sense of capability and confidence?
Coaching can mean a lot of things and happen in a lot of different ways, but "good" coaching, like good teaching has to meet the needs of the person being coached, or it's damaging. That means some people are going to have great experiences being coached and others not so much.
I'm also aware that being a musician and performer requires a certain level of attitude and discipline.
Not "a" certain level of anything. People are different. Skilled artists and craftspeople don't all fall out of the same mold or take the same path to excellence. In fact, many very capable, talented, successful artists and craftspeople follow rather eccentric routes. Reading or listening to biographies is actually a great way to wrap your head around how natural learning works in real life! Because for every artist who follows some stock school model there are a handful who fell into art while doing other jobs, or took a break in the middle to try something else, or dabbled until something changed and they took a leap of faith.
As per some experts it takes 10000 hours of practice to become a good musician.
It's tragic that that gets used to promote the idea that to be good at something you have to slog through a lot of shit you hate. In real life, the real take-away of that particular study was this: passion matters. People who derive joy and pride and fulfillment from what they do, do it a whole lot. Thousands of hours. It's not: "work hard and you'll be great." It's: "doing what matters to you is worthy." Talent matters. Personality matters. Luck matters. Feeling like you deserve to take an hour away from your responsible adult life and to make art, that matters, too. The thousand-hour study is a plea to everyone who's ever thrown up their hands and said "why can't you be more responsible instead of playing those damned drums all the time" to chill out and back off and let artists spend the time it takes to feed their art and their souls.
And this is where 'serious' musicians (or sportspeople or business people etc. etc.) hire coaches.
"Hire coaches" is too narrow a way to describe it. When people want to get better, they look for ways to do that. Sometimes that means finding other people to help... but that doesn't always look like coaching. Sometimes it means finding a new friend, or friends, to hang out with and make art with and/or network with. Sometimes it means biting off more than you can chew and asking everyone around "holy shit, wtf am I doing?" and getting lots of free advice. Sometimes it means watching youtube videos or listening to a million old records. Occasionally it looks like traditional coaching, for a little while, but often that's a very little while - long enough to get over a hump, learn one new thing - a workshop, a class, a weekend with someone who's passing through town.
Can an unschooling parent be a coach as well?
Depends. On you, and how much of your ego is tied up in your kids, on how good you are at watching and listening for the cues that say "this isn't helping", on how adaptable you are to the needs of others. It depends on how your personality dovetails with your kids'. It depends on the extent to which your kids are doing what they do out of joy and how much they're doing it because it's what you want - making themselves who you want them to be to get your love. As parents we have a whole heck of a lot of power to hurt our kids by seeing who they could be rather than who they are.
Something to ask yourself is how much it would hurt your feelings (make you angry) if your kid wanted a different coach, or no coach, or wanted to take a big long break from something they were really good at.
Rather than seeing yourself as a coach, try seeing yourself as a friend. Friends learn from each other. Among artists and craftspeople, learning from friends can be a really wonderful part of the process... but it can also be fraught when our personalities and insecurities clash. Often, life involves discovering which of our friends we can turn to for what kinds of help - there are friends I won't call for financial help and friends I won't call when my art needs a kick in the keester. Imposing the words "parent" or "coach" on the relationship can make the input of a particular friend seem more valuable than it really is.