As folks above said, Tui is about the 100 km out mark. So starting at Porto would be more than that and give you some buffer. I hope your daughter is so enthralled by the experience and the scenery that she doesn't want to miss any of it.
One way to "entice" her a bit might be to start at Matosinhos, and hike the first day along the coast - you can go inland to the central route after the first day, at Vila do Conde. As you may have read, starting the central route from Porto can be a bit gnarly, and you can cut 12 km from the start by starting at Matosinhos at the coast. From Vila do Conde it's a rather long day to Barcelos, or a really short day to Sao Pedro de Rates and another short one from there to Barcelos, and you could take it easy the 2nd and 3rd days by breaking that into two. That could get her into the rhythm of walking and give her confidence that she can do it.