Centre Island Beach is great with younger kids but on a windy day, older ones will have more fun with the waves at Gibraltar Beach.
You can access Gibraltar Beach just a bit west of Centre Island Beach but you can't reach it from the lake shore. Walk west on the paved path and resist the impulse to access the beach from the next few entrances. For the best effect, wait until you see the Trout Pond on your right and a large grass clearing on your left. Walk towards the lake to see my favourite viewpoint. Gibraltar Beach is narrow and long, with a few pebbles. It usually is less crowded than the other beaches on the Islands. There are signs about dunes formation on the premises. If you take Hanlan Ferry, you'll arrive at the red dot shown on our Google map. It will be a 15-minute walk to reach Gibraltar Beach, past Hanlan's Point Beach.
See the blog I wrote two year's ago on the best way to visit Toronto Islands with older kids. It includes Gibraltar Beach.