First off, you need to beat all developer times. I've provided a guide for that here:
After doing that, you unlock the ninja character. The ninja is not only fast, but has the ability to jump infinitely. On the level select, there is a flower you can click on which lets you see all of the flowers you've collected in a sort of hub level. Using the ninja you can keep jumping up in this area until you reach the last secret of Offspring Fling!
Note: This unlocked for me as I went back to the level select after pressing the button at the top.