With an 11-yr old I would choose the Dolphin. They have a 24/7 counter service there called Picabu, Swan has no counter service at all. Dolphin also has an arcade & laundry, neither at Swan. The Grotto Pool area is a tad closer to the Dolphin as well.
I've stayed at both several times & always choose the Dolphin when travelling with the kids for the above-mentioned reasons! But you really can't go wrong with either, one hotel is only a short walk from the other one.