What measures were they using? I suppose not having kids (and probably not having a spouse!) make one's life a lot simpler - you have autonomy, privacy, more spending money, you can eat what you want, when you want, you don't have to clean up somebody else's messes or poopy bottom, nobody smears bananas on your pants right before you go to work, you can be very self-focused and gratify your own needs without having to consider anybody else's. If that's happiness, then yeah, childless people are probably happier.
But long-term, I obviously found that to be an impoverished measure for happiness, or a kind of happiness that one can get one's fill of. Going out for dinner has never made me so happy that I cried because my heart was so full of love for the food or the chef, sleeping in is nice and all, but looking at DS sleeping peacefully with his arm around his plush goose before I come to bed makes my heart swell more than waking up at 10 a.m. ever did, and the joy of watching him learn to draw, run, or help with making pancakes provides me with a deeper kind of happiness than if I did all of these activities solo.