The text does not tell us why Samson went with his parents to Timnah. Presumably, though, it was to introduce them to the Timnite woman and talk about the wedding. It does raise the further question why his parents went with him. Somehow he succeeded in convincing them to join him and arrange for a wedding. They should in no way have consented to his ungodly request.
5 Then Samson went down with his father and mother to Timnah, and they came to the vineyards of Timnah. And behold, a young lion came toward him roaring.