I'll admit it. I only vacuum if we have guests coming over. And whenever I do manage to clear the floors and vacuum, I find that not everything disappears ~ there are always new little spots on the floor. This happened before we had kids too, but of course it's much worse with two little ones running around!
I used to get down on the floor & scrub out the stains with carpet cleaner, water, a toothbrush, and paper towels. Until one day I happened to be cleaning up a fresh spill with a baby wipe, and realized that it cleaned up an old spot too. Now I always reach for the baby wipes, and I've only come across one or two mystery stains that they didn't remove. Here's a little before & after from a recent cleaning day ~
It doesn't matter what brand you buy, but personally I would avoid the 'unscented' type, because ironically, those things really stink! Just grab a wipe and rub at the stain. Use your nails/fingers to really dig in there, and the spot should disappear pretty quickly (some are more stubborn than others, or course!). It does take a little elbow grease, but no more than my old carpet cleaners did. And we always have baby wipes on hand!