Hi,
We used to live near San Marcial, Its a nice quiet area. There is a Netto supermarket nearby for all your day to day stuff ( although can be expensive ) The coastal path is a few mins down the road infront of the Ibrostar hotel and this leads to the marina, We walked it with our 4 year old in 10/15 mins. The beach is a bit further so with all your beach stuff maybe ok there , but you can always get a taxi back.
If you have a car, Papagayo beaches on the other side near San Marcial, (5 min Drive) its 3 euros to get in as its a protected area but the first beach to the right is great for children as its calm and clear water.
http://www.grupoestupendo.com
playablanca@grupoestupendo.es