Can't supply you with an 8 year old playmate but can guaruntee that friends will just materialise
We went several times with just our daughter when she was a similar age and she always made friends with the children on the beach within a minute or 2 of arriving. The thing about the beaches in that area at that time of year is that they are always frequented by the families that own or rent the large pinewalk villas and they seem to have lots and lots of small children .... she will easily find a football game going on that she can join in ( girls and boys ) or a load of children making a sandcastle, collecting shells, crabbing off one of the piers or playing with their fishing nets on the shoreline. Providing she is a strong enough swimmer, there will also be lots of kiddies playing on small inflatable dingies/ airbeds and as there is usually little or no wind to blow them out to sea, its pretty safe and they have a great time.
Oh dear I am getting nostalgic and beginning to think it was easier with an 8 year old than it is with a 16 year old