PB12-NSD is a ported subwoofer. This port is located front top-right and, even with the supplied grille, is quite accessible by any normal 'adventurous' toddler.
One day, I found some small toys hiding about 4 inches inside. To prevent more serious 'breach' in the future, I bought a grille just for this port opening. Fortunately, I found a metal grille that is perfect for the job - a 6.5-in black metal grille from Altronics. And to make it easy to remove someday, I just used a double-sided tape for its mounting. Here it is -
This metal grille has larger holes than the supplied grille by SVS so there should be no impact to subwoofer performance.
Here's another photo of the subwoofer's front (driver + port) -