Everyone hits the wall. Where skating used to be easy and fun, it suddenly becomes difficult, or serious, or both. Among recreational, class skaters, what I observe is that alpha is easy, and beta is cool. Then it gets tricky because gamma is hard, and delta is boring.
And then it gets really hard.
After beta, there are suddenly lots of things to remember--opposition, getting the edges right (on purpose), what to do with that pesky free hip, really not toe pushing. And on and on. The expectations are higher, not only for being able to accomplish new things, but to be able to execute the old things solidly and without excessive instruction. The skills are complex, and it suddenly matters what you know, and not just what you can do with your natural talent. The coach will start nagging about extra lessons and practice time, and suddenly it's not "fun" any more.
You know your kid loves skating, but they're complaining an awful lot. So what do you do?
Some people make actual contracts with their kids-- "I'm about to pay money for skating. This means that you have to come to all of these practices, lessons, and/or classes even if you're tired, or have homework, or want to do a playdate instead." And yes, I mean it about homework. If you and your child can't figure out how to get homework and outside activities to co-exist, you should not be doing the outside activities.
Don't listen to their complaints
Don't indulge the skater's crocodile tears, complaints of sore feet, boring teachers, hard skills or mean classmates. When children test these statements, simply reply neutrally and continue as though there is no issue. If the issue is really serious, the child won't drop it. If the kid is just testing your resolve, the problem will mysteriously go away.
Do listen to their complaints
If the skater suddenly hates this thing that's given him or her such joy, see if there's a real reason. Maybe the coach is boring, or mean, or ignores them. Maybe their skates ARE too small. Maybe a clique has formed that excludes them. Maybe they've been pushed through too fast and it's gotten scary because their skills aren't up to their class level (this is very very common). A complaint that persists, or has a specific pattern, is probably real. Figure out how to mitigate it.
Let them have fun
The last 5 minutes of any class should be extras that aren't in the curriculum. It might be putting on a little exhibition, or games, or coming up with a program, or learning something from a higher level. Even a high competitive child should be allowed to have some fun interspersed with the work. Private lessons probably aren't the place for this, but your skater should have at least some control over their own practice, and shouldn't be yelled at for taking a few minutes every now and then just to have fun.
Give them something else to do
It's okay to take vacations from skating. A week, or, frankly, a year. (If it's a year, don't expect to come back in at the level you left.) Don't skate every day (even competitive skaters). Have another activity that the skater enjoys.
Make sure they skate enough to improve
One of the biggest things that takes the joy out of skating is when you can't progress. If you are constantly missing lessons, or not skating enough for your level, your peers are going to start leaving you in the dust. This will make anyone feel bad. Believe it or not, O modern parents, you might have to ask your child to choose from among the 19 different activities that they participate in in their relentless pursuit of admission to Stanford.