both resorts are lovely , not stayed in Tossa but visited for the day, so cant comment on things to do for your son . Again Blanes is a nice resort , we stayed there about 10 yrs ago and the children were much younger and there was nothing for them , but things might have changed since . I would say Lloret , Callella or Santa susanna would have more to keep your son entertained . I'm sure you will have a good holiday which ever resort you choose

the royal beach in lloret will probably tick most of the boxes for you. fenals area is fairly flat and there is bus stop(and taxi rank) outside the hotel.kids club and daytime activities are reportedly very good.
blanes may be a bit quiet for an 11 year old and i think tossa may be much the same -but i dont like tossa anyway.
in calella most of the modern hotels are up a slight hill at one end of the resort, but have a noddy train running all day to take you into the main town/beach which has a very flat shopping area.hotels on the south side of the main road are all predominantly on level ground.

I agree that Fenals is fairly flat, although I would recommend the Aquarium Hotel. It's in a quiet area, just off the main Lloret road and with easy access to bus stops and the local shops. It's about ten minutes stroll to the beach.