If the minor is accompanied by parents, then ID cards are still a good idea if the child is going through security, so that it matches the minor's parents identification information. However, the presence of a parent makes the need for the minor having ID on them less important, as they should be able to stick with their parent who will vouch for who they are and where they are going. In the event the parent and minor are somehow separated, the more ID they are carrying the better.
When a minor is traveling an ID card should be printed up for them to carry, if they need to show it to someone who works for the transportation service or an authority figure. On the card should be the minor's name, a couple helpful contact numbers and where they are from. It may also be a good idea to have a card or piece of paper with the minor's travel itinerary so that all the information as to where they are coming from and where they are going is laid out for the authorized person to help them.
As with adults traveling overseas, minors need proof of their citizenship when going to and from countries. This could be a passport, but as passports are valuable commodities and easy to lose or have stolen, a folded copy of the minor's birth certificate might be the best thing for them to show proper identification. This must be raised and embossed with a registrar's signature and stamp from the registrar's office before it is valid. If a parent is traveling with the minor, some sort of paperwork should be presented to show evidence of a relationship between the minor and parent, be it a court order document or the birth certificate itself, matching the parent's last name and minor's last name.
