I have purchased a lambs wool cot underlay by baby rest for my 7month old. How safe are they to use?

Red Nose recommends that you use a firm mattress for baby.
if the woolen underlay takes away from the firmness of the mattress, then it would not be recommended for use.
Never put soft bedding under the bottom sheet, such as a sheepskin, as this makes the sleeping surface too soft
https://rednose.org.au/article/what-is-a-safe-mattress