Hi,
do not worry. After Porto there are plenty of albergues, so it should be possible to walk shorter stages if necessary. If you do not pre-book, you will be flexible. Only if you start rather late in the morning and arrive on your next stop in the late afternoon, it may be difficult to find a bed and pre-booking could be a relief.
Between Ponte de Lima and Rubiaes (18 km) there is no bus service, but now a new albergue has opened in between. In the other sections there should be bus services, though probaly only twice a day. Taxis are about 1 € per km. But I do not think that you will need these options.
Young people like your daughters will recover more easily. Go to a bar, have a break, a coffee or an orange juice and on you go!
BC
Alexandra