My ds who's 8 months has the same if not a muslin square one of those very thin and small holey blankets. I always wondered the same thing but when ever i check on him it's always away from his face. i think if they can hold thier head up they wont suffercate themselves. But for ages i did worry and was checking on him all the time. he's had it now for a couple of months and wont go to sleep without it now, luckily i have plenty of them so i can wash them.