I see nothing wrong in buying sweets for one’s family in occasions such as the Prophets’ birthday provided that one does not believe it to be a sunnah, a ritual, or a religious obligation in itself. It is only a tradition that some communities used to do to show their happiness in such occasions.
I myself think that it is a chance to make one’s family feel happy and love these Islamic occasions. Let’s seize the chance of this occasion to make our kids love the Prophet by acquainting them with his character in a good and lovely way.
Don’t worry—dear brother. Eat from the sweets, thank your dad for it, and give gifts to your friends in such good occasions, if you could, with good intentions.