Putting ice in your back pocket is probably better than nothing, but not much better. For that to work, you will want maximum contact between the ice and your circulatory system (your body's natural radiator). It seems like most of that ice is just going to melt and drip on the pavement.

It's probably a better idea to go to an ice cream shop in town and buy a slushy drink to take internally. Then fill up water bottles with ice and water and put that in your back pocket. After the ice melts, you can drink the water in the bottles.