The IV serves 2 purposes:
- Re-hydrate the patient.
- With the needle in, the kid is nowhere going to run around or being bothered by other children and as such given the rest to recover.
It's a bit the same principle when the kid does have a big plaster on the forehead, shouting "Leave me alone". Or Khmer women who wear a knitted bonnet for 2-3 months after they gave birth.