Should my daughter carry her passport or is it enough with the Family Book?

I am a U.K. citizen and my wife is Spanish. We have planned to travel abroad, out of the Schengen area, with our five months old daughter, to visit my parents who have not met their granddaughter yet. She has Spanish nationality as well. A friend of us has told us that the Family Book (Libro de Familia) is not enough to prove that she is our daughter, and that we need to obtain a passport for her.

Is that true? If that is the case, we have a problem, as I have requested an appointment to obtain it, but the date is after our departure date.

Minors, like any other person, need to obtain a DNI / passport to travel abroad, being able to do it by being included in the passport of one of the parents or by obtaining their own passport, since the libro de familia does not have any value as identification document as it does not contain photographs. Nevertheless, if the problem is that the date obtained is after the trip, you do not need to worry. You can go to the police station corresponding to your address and bring with you the plane tickets or travel reservation, and the appointment they gave to you. This way, as you can prove that you need to obtain the passport and the date given is useless, they will process it instantly.