CP does not have an escort service for unaccompanied minors. However, if the minor is mature enough to travel alone, CP does not impose any restrictions. Minors travelling alone with a ticket travel under the responsibility of their parents (or other legal guardians).
Children up to and including the age of three, when accompanied by adults, are entitled to travel free of charge, provided they do not occupy a seat. If they do occupy a seat, they need a half-price ticket. 
If they are over three and under thirteen, they also travel for half price on Alfa Pendular, Intercidades, Regional, InterRegional and Lisbon, Porto and Coimbra urban trains. 
For more information, see the Children's discount.