A suitable temperature for a child's sleeping environment is 16-24°C (61-75°F). Some factors that may affect the temperature under the duvet are climate, room/body temperature and amount of garments. The examples below are just a guide for how you can dress your child at different room temperatures with a duvet that has a TOG value of 3.5:
At 18-20°C (64-69°F) – short-sleeved onesie.
At 16-18°C (61-64°F) – long-sleeved onesie.