The problem is the kids DO KNOW how loud it is: When the cat's away, the kids R gonna rock the house no matter what anyone says. (Rock On Kids !)
Get them beefier outdoor speakers or make it a zone and set up a keypad or volume control that has a "preset max volume" setting for that zone. My control unit has that function and no matter how many times the UP arrow on the keypad is pressed the local volume will not exceed the preset.

Perhaps the owner will spring for smallish PA style "outdoor event" speakers. They won't be audiophile grade but do you think the kids will know the difference while they splash in the pool/yell/scream? What the kids really want is BASS: Maybe you could install (sell) one of those subs that you bury in the planter - they'd probably still just crank it up.