Should a child under the age of 16-18 months be wearing a jumper under their short sleeved sleep suit when put to bed in a cot room Should a child under the age of 16-18 months with no sleep suit be put to bed with a jumper on in a cot room

Red Nose does not have specific information for each baby/child in terms of amount of bedding, but recommends sleeping baby in the amount of clothing dependent on room temperature.
However, it is usually recommended that a baby/child wears light clothing when sleeping, adding or removing extra layers as necessary, depending on the room temperature.

From ACECQA - Safe bedding
Light bedding is the preferred option
https://www.acecqa.gov.au/resources/supporting-materials/infosheet/safe-sleep-and-rest-practices

Red Nose has the following general information.
Dress baby and use layers as you would dress or use layers yourself: to be comfortable, neither too hot nor too cold.
https://rednose.org.au/article/bedding-amount-recommended-for-safe-sleep

Red Nose does not recommend a specific room temperature for baby’s sleep. This is because there is no evidence to show that maintaining a specific room temperature prevents sudden infant death.

As long as baby is put down to sleep on their back, and that baby is dressed appropriately for the room temperature – not overdressed or under dressed – with their head and face uncovered, you can feel reassured that you are protecting baby from overheating.

We don’t believe that it’s necessary to use a room temperature monitor, or to leave the heating or cooling on all night, as long as baby is dressed appropriately for the temperature of the room – not too hot, not too cold.
https://rednose.org.au/article/how-to-dress-baby-for-sleep