Hello Just wanted to confirm if there are any materials for bassinet or cot mattress protectors that shouldn't be used for safety. Eg mattress protectors that have some plastic coating. thanks!

Red Nose does not recommend (or “unrecommend”) any specific type of material for a mattress protector.
Red Nose says to avoid the use of a soft/padded fabric that may compromise the firmness of the mattress.

...always make sure that the waterproof mattress protector is strong and a tight fit.
Never put soft bedding under the bottom sheet
https://rednose.org.au/article/what-is-a-safe-mattress