Understand what a piggy bank is. It's an object, maybe 6-8 inches long, in the shape a pig. It is hollow, with a slot in the top through which to deposit money.
The earliest piggy banks were made of clay/porcelain and had no provision to open them up to retrieve the money inside -- the theory was that you saved money until it was full, then smashed it open and used the money for some good purpose or deposited it in the (financial institution style) bank. If you just needed a few coins for bus fare or some such you might try using a dinner knife to fish money out through the slot. Later versions of the bank, of course, came with some provision to remove the money through an opening on the bottom.
The idiom "robbing the piggy bank" could be interpreted several ways, but generally implied using the money for other than the originally intended purposes. And, if the image of the un-openable ceramic unit is in mind, it could imply a certain violent nature to the "theft".
Of course, in today's day and age many people have never even seen a piggy bank in use, and few are aware of the issues surrounding the ceramic unit. So the meaning and implications of the idiom are very vague, unless context provides a foundation.