The real advantage to adding swimming to a running program is that the time spent swimming aids recovery. The increased blood flow, without the damaging effects of the impact pounding from running, enhances and speeds the healing process. Plus, the kicking motion loosens the lower leg muscles, reducing muscle tightness and the corresponding stress on tendons and other connective tissues.
The added total body, and especially core, strengthening that results from swimming is also a plus.
Of course this assumes the athlete swims with proper stroke technique, something that does not come naturally. Otherwise, they may as well just spend the time water-running, or even just getting a massage.
The people who can best take advantage of this are former age-group swimmers. So maybe the best way to prepare your kid for a future as a runner is to start them off as a swimmer.