Is it ok to use a fitted wool underlay under a fitted sheet in the cot? It feels quite firm under the sheet. As it gets cold, just looking at ways to keep things warm.
Red Nose recommends the use of a safe mattress, which is firm, flat, and in good condition. The information statement, “What is a safe mattress?” states, “Never put soft bedding under the bottom sheet, such as a sheepskin, as this makes the sleeping surface too soft.” You can find this information at https://rednose.com.au/article/what-is-a-safe-mattress
For suggestions on the amount of bedding to use to keep baby warm, please see: https://rednose.com.au/article/how-much-bedding-does-baby-need and