Hun, it's phonetically easier for children to say dad, I'd guess that about 90% of first words are dad daddadadad.

I know a few children (including my nephew) that don't have a father in their lives AT ALL and their first words are still dad.

So stop it! Nothing to do with him, if he wasn't even in the country bubs would have looked at you, your sister or the wall and said it

xoxoxo