In my opinion, Golding does this to model that these little ones are innocent and children. The idea of play models that they are still developing and not capable of responsibilities that others will have to carry for them. Innocence is demonstrated in this as well because they did not choose to be here, and this play shows their oblivousness to the serious nature of the new situation of being stranded on this island without adults. It is almost as if these small kids expect that the adults will come and find them and take care of them.
We find the beach and ocean to be calming. These children are highly relaxed. This will certainly be contrasted later with the fears little children are capable of possessing. The entirety of the setting contrasts with the predicament the boys are experiencing. This moment at the pool just exaggerated that. Read about setting in the following link.