In my opinion it should be accepted, but if considered strictly then "daddy" is an endearing name for "dad" or "father", similar to how "isi" is an endearing version of "isä".
For learners of Finnish it can be difficult (just as it is for us natives) that there are multiple endearing names in use: isi, iskä, iska... depending on your roots and the region you live in. Currently the course accepts only one or two alternatives.
I disagree. In English dad and daddy are both endearing names for father. The only difference is cultural, down to each family's own culture as well as the larger regional English and class culture too, there's little definable difference. Duo shouldn't be separating two such well used terms, both should be accepted.