yes, depends on how old they are...........if they are just teens then there is a arcade type park with pool tables, games, some rides and the like.
if they are older then dunno.......cala llonga is relatively small and i'd imagine it'd be quite difficult to get into that much trouble. let them go for a few drinks or somethin.
i think there is an aquatic circus on in sta eularia (5 mins down the road by car)..........that's one night boxed off.
sta eularia itself is also good as a place to go one night, likewise ibiza town. you could maybe let them go off on their own and arrange a meet up later on. lots of street entertainment, stuff going on in both those places