On Isla Canela there is no bank, but there is a cash point machine in the marina (in the middle between Sugar Reef and Pasta restaurant). The nearest bank is just over the bridge (past the football ground), on the right hand side.
The nearest pharmacy is in Punta del Moral, all other shopping is in Ayamonte. A number of bars/restaurants have closed in the Marina due to lack of toursits and I understand quite high rental charges on the units. So they will find a number of places shut in the Marina.
The majority of apartments in the Marina are Spanish owned and as previously said, Spaniards don't holiday until July/August.
I understand what your parents are saying, it can very much feel like a ghost town, it was like that a couple of weeks ago over Easter, and to be honest June, July & August are heaving down there. I can honestly say that it appears to me that every other apartment at the Marina either has a for rent or for sale sign outside.
Having said that, I hope that they have a great time, and have the opportunity to go into Ayamonte and enjoy a coffee in Plaza Laguna and watch the world go by. Also if the opportunity arises, I recommend the calamari and mixed salad at the Bombadilla chirunguita, which is at the other end of the beach to the Rui Atlantico.