Questions & Answers about Afmælisbarnið blæs á kertin og hlær hátt.
Icelandic often uses the definite form when you mean a specific, known person/thing—similar to English the.
Here, in a birthday context, there is typically one salient person: the birthday child/person, so Afmælisbarnið is natural.
Indefinite would be possible in a different context:
- Afmælisbarn blæs á kertin. = A birthday child blows on the candles (more generic/odd in a normal birthday scene)