Parents are concerned that children who have trouble in school will 'fall through the cracks' in the school system. [=will not be given the help they need; will graduate without anyone noticing that they are unable to do the work]
The program is meant to help workers who may have 'slipped through the cracks' [=who may have not been included in other programs] because of their age.
You ought to 'get cracking' [='get going; get moving'] on that assignment. [=you should start working on that assignment right away]
(..) - - - - Ooh! Shrek! Did you do that? You gotta warn somebody before you just crack one off. My mouth was open. Believe me, Donkey, if it was me, you'd be dead. It's brimstone. - We must be getting close. - Yeah, right, brimstone. (..)2001 Shrek