You know, there was a thread about this a couple of years ago that would be worth reviewing if you can find it.
To me, the short answer is that the question itself is based on a false premise, i.e., that the "work" in "work ethic" means "leave the house for 40-80 hrs/week to work for pay." That's just wrong.
Kids watch how you live and what you value -- and they determine what you value by watching what you do. If they see you regularly taking care of the house, cooking food to put on the table, spending weekend days doing chores around the house or some sort of productive hobby, maybe volunteering for some cause that you care about, they will learn that you value taking care of stuff that needs to be taken care of and using your time productively. If you are open with them about your finances, your salary, your savings, etc., they will learn that working at a paid job can be an important value, too, because it gives you the $$ you need to do all of those other things. But it is only a part of the picture.
Moreover, paid work is not even the most important part of the picture. I mean, if you work 60 hrs/week, then watch TV every night and weekend, are you teaching your kid the value of work? They're not there to see you put in the effort when you're in the office; all they're probably learning is "work is super exhausting, and boy I don't want to do that." If you want your kid to learn a work ethic when you're out of the house doing invisible stuff for 60 hrs/week, you're going to need to spend a lot of time at home talking about what you did that day, what was good, what was bad, the challenges and successes and disappointments -- they need visibility into what "work a paid job" actually means.
But even then, kids get 10x more from what they see than from what they hear. So rather than chase a red herring ("how can I explain to my kids how important it is to work when I don't have paid employment anymore?"), focus on what you actually do for all those various hours when you're around your kids. That's what matters. Are you engaged, are you challenging yourself, are you responsible about getting things done even when you don't want to, do you put effort in to making the world a little better for someone else, etc. etc. etc. That's what "work" actually is -- and that's the kind of "work" kids can see, understand, and get excited about.* And, conveniently, it's the kind of work that you can in fact do even more of once you FIRE and no longer have to worry about that pesky paycheck.
*As my kids are moving toward late HS/late college, I have shown them both MMM's "Shockingly Simple Math." DS looked at me like I was out of my mind -- to him, the reason he is going to go to college and get a job is because he wants to build robots and learn cool stuff; saving huge amounts of money to quit that sooner was completely counterintuitive, because he's in it for the work itself, not the money. I had to reframe the issue as having the financial freedom to take the job you want vs. the job that pays the most; then it clicked in.