Not exactly a secret, or wildly fantastic, but a good trail. It has several elements that make a hike kid-worthy. Stairs, bridges you can spit from, and water elements. It's a state park, and requires a $2 entrance fee per car. It's not too far from civilization (just a mile or so from the Hunter's Crossing Publix at 43rd and 53rd). Bathrooms and water fountains.
The main attraction is a 120 foot deep sinkhole. "Sinkhole" doesn't sound very attractive, does it? But the temperature drops as you go down the stairs (something like 250 of them?) and if you imagine real hard (don't close your eyes as you do this, or you'll end up tumbling down a lot of stairs) you can imagine you're hiking in a mountainous rain forest.
It's all ferny and lots of springs spew from the edges and tumble down hobbit-hole terrain. Very pretty. Your calf muscles will pay for the trip later, though. That's a lot of stairs!
There's also a walking trail that goes around the property. My two-year-old walked it easily, and we saw fence lizards, lots of five-lined skinks, and awesome funguses along the way.
Anyway, the Devil's Millhopper will be a very pleasant hour and a half, and can be found on state road 232, also known as Millhopper Road. It's actually the far western extension of 53rd Avenue. From the intersection of 53rd and 43rd (that intersection has the Hunter's Crossing shopping center with the Publix), go west, on 53rd/Millhopper. It will be just a couple of miles down, on your right.
(blogger is once again being persnickety about photos, so there may be more later...)