I've never seen the paper version, but I have visited the online version of mental_floss many times. It is, as the site's tagline suggests, "Where Knowledge Junkies Get Their Fix." Children's writers can find ideas galore to pursue.
Earlier this week I found a quiz on the site, which every children's writer should take. I'm ashamed to say I failed horribly. It's The Very Hungry Caterpillar quiz. Check it out and see how you do.