I have been having this problem alot. No matter what genetics a born in game baby has, when it is aged to toddler it has black hair.
I experimented today. I made a test family The father with Red hair and the mother with blonde hair. They were CAS families so there is no funny back DNA somewhere. They had a baby and it was born. It had blonde eyebrows (making me assume blonde hair) and blue eyes.
when the child was aged to toddler though it had black hair. The brows were still blonde though. See the picture.

does anyone know what could be causing this?