I can't install AdBlock in a supervised Chrome account

It isn't possible to install extensions in supervised accounts.

There is an alternative to using supervised accounts, depending on how old your kids are. If they are 13+ (the minimum age for a Google account), you can create a Google account for them and install AdBlock in Chrome on their computer. Then you can use the parental controls on your router to limit the places and times your kids can access the Internet. Check your router manual for instructions.