The secret route is easy to find, but hard to take.
Use a trampoline to find it.
The place is before the end of the mini-race (if i remember correctly),
It's the first place with trampolines, so when you see a green trampo. jump on it and you'll be in a "tube", at the end, you'll obtain a hundredsparkle.
Better than words, a video (credit to: ps4trophies)