most of the hotels tour companies offer this, but as part of a tour of barcelona. they normally do an afternoon tour that encompasses this. normal price for tours apply -i would think around €40 each, but can't be sure.
there are a few places in malgrat that offer day tours, situated around the planamar hotel and the rosa nautica, and these tours tend to have a pick up at the rosa nautica and are cheaper than the reps tours.
you could just go to barcelona on your own for the day, a lot cheaper, but you should check the train timetable carefully as if i remember correctly the fountains show finishes around 10-10.30 and the last train back is not very late.
remember also that pickpockets operate in barcelona at all the main tourist sites, and the fountains show is also popular with them.

Thank you so much for your reply will pass the information you have given on to my daughter