Easiest way to think of this is that this is Gatorade. Sure everyone thinks that sports drinks have glucose for the energy (which is also true) but they also contain sugar because the Na/Glucose co-transporter in the small intestine helps drive electrolyte intake. Without glucose, you don’t pull in sodium nearly as efficiently in the gut and the first makers of the Gatorade formula at UF found that once they gave glucose and electrolytes instead of just water to the football team during practice, they didn’t get as dehydrated and their electrolyte balance was a lot more stable.

Glucose is co-transported into enterocytes of SI via sodium

Kid probably had cholera -- giving Na+ and glucose takes advantage of sodium-glucose cotransporter thereby replenishing electrolytes and energy.

The patient is dehydrated. Need to give water to rehydrate.

Water follows solute. If sodium is added to the solution, water will follow the sodium. Now, enterocyte uptake of sodium is mediated by Sodium-Glucose transporter. Hence, the solution needs to contain sodium and glucose in order to have the enterocytes take up the sodium.