Is it safe to use Organic Cotton Flannelette Cot Fitted Sheet for babies under 12months?

Red Nose has no information about type of fabric used for sheets or other bedding, apart from recommendation for light weight bedding (in particular blankets)
Ensure baby is placed for sleep on a firm & flat surface.

How old is your baby?
Remember, to consider overheating when deciding on the type & amount of bedding for baby (particulary baby less than 6 mths of age.)
https://rednose.org.au/article/protecting-baby-from-overheating-during-sleep