Congratulations on the visa, it's a wonderful feeling. We arrived last October and chose Seaford Rise and Moana to settle. We have boys the same age as yours, one even shares the same name. There are good road, bus and train links to city but we work at Flinders and locally so this is a good location for us.
We have the Wine Region (Mclaren Vale) behind us 10 mins bike ride, beach 5 mins bike ride in front of us, hills and city also within 20-30 mins drive. We love it here and the boys have settled well. We stayed in Moana Caravan Park when we first arrived, for 6 weeks, then rented here and have just bought our first house.
Wherever you choose you are never far from the beach, depends if you want to be within walking distance (that was important to us) or driving distance.
In Summer we went down nearly every night after work/school and it was superb. Boys have got involved in cubs/scouts, footie, cricket, soccer, basketball either here or at Port Noarlunga, 5 mins drive away.