Camp Half-Blood is a Greek demigod training facility located on the Long Island Sound, directed by Dionysus, (Mr. D.). This camp is described as being the only safe place for half-bloods.
Percy says that the camp part of the beach is invisible to mortals, except the ones who can see through the Mist. Monsters and mortals can not enter, unless permitted from the inside.
So where did your kids go to camp this last summer?