I don't really believe happiness exists without kids. Happiness can exist when you are a kid and it can exist when you have kids. It cannot exist otherwise. You can be satisfied at any point of your life, with or without kids, but you can never be happy.
I remember being happy --- I was a kid. I remember when it stopped --- gradually when I became a teenager and completely when I became an adult. I had a career, I had money, more than I could spend. I had a loving wife, better wife than I ever expected to have. I lived in a free country. I was passionately interested in a myriad of subjects: Films, books, sports, programming, science, I had a lot of hobbies, never stopped being busy. All those hobbies I had since I was a kid. But I realized that I don't see them the same way I saw them when I was a kid.
Sports, I hated long-distance running as a kid, found it tedious and grueling. As an adult, I would run and run and run, run through pain, through exhaustion, became sick with over-training, developed anemia, had multiple injuries, had pain almost every day, ran through injuries because I didn't want to lose fitness, reached almost 20 BMI, was frail and weak. What for? Nobody cared, not even me. It was just an obsession, looking for something that wasn't there.
Science, it was no longer the wonderful curiosity-satisfying endeavor that it was when I was a kid. It had become stressful research, anxiety to learn as much as possible, guilt and feeling of worthlessness when I couldn't really understand something after studying it for a period of time.
Films, I would watch 100-150 per year. Had a film-club subscription and watched pretty much everything they put on. Went to film festivals and watched 30+ films in a week. Watched something at home regularly. When you watch so many films they become tedious and you hate 90% of what you watch but I had to keep doing it because, what else would I do with all the free time I had?
I realized that there's nothing wrong with all those activities, what's wrong is what I expected from them. I wanted too much from them. I wanted them to give meaning to my life which was at a point when no meaning could be extracted by such activities.
And then I had kids. It was stressful, yes, it was tiring, yes, but oh boy --- no doubt about it, I was happy. I wanted to run out on the streets screaming from my happiness. I could not get angry any more. Everything that happened to me I just smiled and went on. And 6 years later I miss every time I awoke at night, every time I held them screaming. I am absolutely and positively sure that there can be no greater happiness in this world than the one I felt back then. I realize that I can have another kid now but it won't be the same.
And it all feels like it was one second ago. It all passed me by in an instant. Now my kids are old enough to do almost everything by themselves. Soon they will know how to read. I now do all the activities I mentioned above. But no obsession. I run enough to stay fit. I started weight-training which made me feel and look better than ever. I study science with a smile and with an understanding that I will never understand everything.
Everything has meaning now.