Subbing in a game can be as simple as getting an injured player off the field or as controversial as positioning kids in formation to best counter their opponents. Playing-time is a big issue in youth sports, but the number one responsibility of the coach is safety. This includes appreciating how children physiologically react to their environment, accounting for the heat index, and knowing the condition of the players.  The younger the player, the less developed the body's cooling system. Sweating is the way for the body to cool itself off. The sweat seeps out of sweat glands located all over the skin, but these begin to mature with puberty, so young players, especially under 12 years of age, need more water breaks to properly hydrate and cool their body temperature since they sweat less.  Kids also gain heat quicker than adults and tend to lack recognition of when they need to hydrate. Usually at water-break in practice, one or 2 kids will say they're fine and want to stay and play. This may be that they ARE properly hydrated, or more realistically, it may be that they don't recognize their body's need for hydration. Either way, it is essential that Coach makes sure all players get their fluids. Wether or not a game has scheduled water breaks in each half, in hot and especially humid weather, the coaches need to insist on them. FYSA (Florida Youth Soccer Association) recently established a policy for mandatory water breaks in age groups 12 and under...and in 13 and up ages, breaks are dependant on a charted heat index. Once the players rehydrate, they will feel refreshed, be more effective, and have a lot more fun in the game.

References: 

-Personal knowledge through experience as well as studies at UWF and FIU in Sports Management and Sports Administration

-PEDIATRICS Vol. 106 No. 1 July 2000, pp. 158-159

AMERICAN ACADEMY OF PEDIATRICS:
Climatic Heat Stress and the Exercising Child and Adolescent

Committee on Sports Medicine and Fitness 

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