According to the Centers for Disease Control and Prevention, infants and children younger than four-years-old are among those at greatest risk for heat-related illness.
For babies under six months, doctors suggest: keeping them out of direct sun, have them hydrated prior to going out and give them a drink every 20-30 minutes.
For older kids, anticipate and plan ahead. They, too, need to drink before going out. Take frequent breaks to rehydrate. And avoid anything with caffeine. Also, don't forget that while water is usually sufficient, sports drinks also help balance electrolytes.