It was because you need the definite article : Al suo bambino.
"Bambino" is not one of the family members for which the definite article is not used.
I was marked wrong for "a suo bambino". But I thought family relations didn't use the definite article?
I hope this helps. Truthfully it's a bit confusing :)
Sure is...I put "a sua bambina" for a female baby and was also marked wrong even though it was an option in the word clues!!!