3 year old has asthma so often suffers badly with a night time cough. Among other things, we've found an adult pillow helps him. Before the age of 2 he had a very thin toddler pillow. I thought he was robust enough now for a normal pillow, especially if it helps him sleep. But someone said he shouldn't, it's dangerous. Is this the case? Raising his mattress doesn't help, he just rolls down and sleeps at the opposite end of the bed.