I guess your stay is near Besant Nagar, From Besant Nagar move to little mount or guindy(You can get lot of buses to guindy, not sure about little mount). From Guindy, you can take metro to Alandur. Take an auto from Alandur to St thomas mount shrine. You can ask them to drop at foot hill from where you can climb through steps(you can also go till top in auto, but they might charge more).
It is good place from where you can see entire Chennai airport, with lot of take off and landing in evening hours. Kids will love it. Make sure you reach shrine by 4-4.30pm.