The type of yarn you use is very important. If someone has no hair, their head will be very sensitive. Yarn that doesn't seem that scratchy to you will feel scratchy on their head. A good way to test that is by rubbing it against your cheek. If it feels scratchy on your cheek, try another yarn. You can't get too soft, so the softer the better.
It's also important to use yarn that can be machine washed and dried. People will be wearing these hats everywhere; they need to be able to clean them well. Unfortunately, natural yarns are often scratchy and not machine washable, so for the most part, go for acrylics.
The other important part is that you should try not to use yarn that's too thick. The thicker the yarn is, the warmer it is, and people will be wearing these hats all year round. They will overheat if the hats are too warm, since they will be wearing them during the summer and inside.
I found that baby yarns and soft acrylics are the best type of yarn. Babies have similar limitations with yarn choice as these people. It needs to be dryable and soft for them too. Just make sure that it's not too thick, because babies do need warm yarn sometimes. For acrylics, just make sure it's not too scratchy or thick.
Some of the yarns that I found were good were:
Bernat Softee Baby
Classic Chunky Impressions-with a less heavy pattern
Zarella
Laines Du Nord: Baby Star and Baby Cover
