Recent postings on this website have indicated that there is some confusion about the use of Länder-Tickets (all-day passes for a specific Land) in Germany. Some people have stated (I don't agree with them) that the descriptions on the English side of the German Rail website are vague. They used this supposed vagueness to justify using a Bayern-Ticket in a manner that is not explicitly permitted. If they really believe it is vague, they should make an attempt to find out the correct interpretation, not just use the one of several interpretations that best suits their needs.
Information is provided in English on the German Rail website for the convenience of the traveler. However, the site has a disclaimer regarding the accuracy of the information. The only legal source of information is the AGB (Conditions of Carriage) of the Bahn. That information is in German, but, hey, you are in Germany, and that is the official language.
After carefully reading the German language AGB (Conditions of Carriage), I will try to clarify it, at least as far as I understand it.
First, some background. A German state is a Land (pl. Länder). Other than the city-states of Bremen, Hamburg, and Berlin, every German Land has a Länder-Ticket, either by themselves, or in conjunction with other states. The three city-states are included in the Länder-Tickets for surrounding states (For example, the Niedersachsen-Ticket includes Bremen and the inner two zones of Hamburg.
The primary use for a Länder-Ticket is for up to five people (6 years of age or older) traveling together. For most states, children under 6 do not count, they're free. Also, for most states the Ticket is sold for the first person, and up to 4 others pay an extra charge each. Exceptions: Hessen has only one ticket for up to five people for a flat 33€, while Nord-Rhein-Westfalen has a ticket (Schöner-Tag-Gruppen) for up to five people at a flat rate of 42€ and also a ticket for a single person at 29€.
Most of the states also have a secondary way to use the Länder-Ticket, which is one adult and all of that person's children and grandchildren under 15 years of age.
The AGBs for each of these states all have the following wording,
“kann genutzt werden von:
3.1.1 bis zu fünf gemeinsam reisenden Personen oder
3.1.2 einer Person mit beliebig vielen eigenen Kindern bzw. Enkeln bis einschließlich 14 Jahren (sog. Familienkinder“) und einer weiteren Person”
can be used by
3.1.1 up to five people riding together or
3.1.2 one person with any number of their own children or grandchildren up to including 14 years (so-called Family children) and one additional person
Note, it says “or”. That is one way or the other way, not a combination of both.
The question I can't answer, and it wouldn't come up often, particularly for visitors, is, can a parent bring all of his/her children under 15 AND all of his/her grandchildren under 15, or only children OR grandchildren? I think it's the latter.
The AGBs for Schönes-Wochenende- and Quer-durchs-Land-Tickets, offers of the Bahn, have the same wording.
The additional person can be a spouse, an adult child, or anyone else. I don't think the additional person's children/grandchildren are included in the “family children”.
Some Länder AGB give additional clarification.
Niedersachsen specifically says the “family rules” are not for Niedersachsen-Tickets for 3, 4, or 5 people.
Nord-Rhein-Westfalen allows one or two adults who purchase the 42€ Schöner-Tag-Ticket-Gruppen ticket (the one for up to five people) to take along their children and grandchildren. It must be the Schöner-Tag-Gruppen ticket, not the Single ticket
Finally, Hessen, which only has a flat fare ticket for up to five people says that up to three children under 6 are not counted and can come along for free. Older children, 6 to 14, are considered as “mitfahrer” and count against the total of five.