Yes. Getting from Portugal to Spain is not as convenient as one would think. I found that out my first time on the Camino. There are not a lot of trains or buses going between the countries. If your daughter has her heart set on the
Camino Frances, then I would suggest looking at a cheap airline, such as Easyjet or AirEuropa (just look on Kayak) from Lisbon to Madrid, and then catching a train up to Leon. (It would be well worth it to spend an afternoon and night in Madrid.)
I wouldn't start any earlier on the Camino than Leon. Leon is the end of the stretch across "The Meseta" which is a long flat farming area (picture a Spanish "Nebraska"). It's really great if you have all the time you need, and I do love it and will be on it in a few days; but if that is the start of your camino, it can be a bit disheartening after a couple of days.
Your daughter is going to love it. A couple of years ago I took my 18 y/o nephew on the Camino for his graduation (along with a few days in London, Paris and, of course, Amsterdam). He LOVED it. You are a great parent. I hope you enjoy this special time with your daughter.
Edit. Also, don't forget while planning that Portugal and Spain are in two separate time zones.