Final answer:
The child's symptoms of a red face and fussiness after playing outside on a hot day, with a normal temperature, are indicative of a response to weather and activity, not illness.
Step-by-step explanation:
The condition of the 6-year-old child who has been playing outside on a hot summer day and presents with a red face, fussiness but a normal temperature, is most likely explained as a response to the hot weather and physical activity. The red face and warmth can be attributed to the body's natural cooling mechanisms, such as vasodilation, where blood vessels expand allowing more blood to flow to the skin to release heat.
Since the child's temperature is normal, it suggests that the body is effectively regulating its temperature despite the external heat and exertion from riding a bike. Additionally, being fussy can be a common reaction to feeling overly warm and possibly dehydrated, so ensuring the child is well-hydrated and allowed to cool down would be appropriate measures.