As you can see, you will generally need one ball of yarn or less for making hats for small babies and possibly two balls of certain yarn types for hats for larger babies. (thespruce.com)
Get them to make a hat with ideas they will work towards and learn with a growth mindset. (proudtobeprimary.com)
Note to self - do not make hats too small, or they look ridiculous! (theartyologist.com)