In reference to children attempting to dig to china in their sandboxes, the moral of the phrase is that you shouldn't stop trying hard just because you've achieved /something/. For example, just because you've graduated and you have a steady job and you're doing fine doesn't mean you shouldn't go to college and aim for a degree. It's constantly striving to be better that bring people the greatest of success.
Also - just don't shit in the sandbox, that's just rude.
Jessica: "Don't shit in the sandbox just yet! I hear your boss Roger is getting ready to fire that bitch of an assistant he has, and I'm pretty sure you're next in line!"
Kathleen: "Shit in the...what? What is that even supposed to mean?"
Jessica: "I don't know...my grandpa used to say it all the time."