Disclaimer: The following paragraphs are not to be used as a recommendation, but describes what we experienced. Others might encounter difficulties if for instance one gets selected for a check by customs.
Well, we didn't have time to get the passport, and didn't want to spend a lot of money on an emergency passport, so we went ahead and tried.
The following is true for Norway-Germany, and Germany-Norway:
Airport security and check-in: No problems what so ever. The airline staff did not ask for passport for any travelers.
Airport security and customs upon arrival: No problems what so ever. No one even looked at us.
We did bring the birth certificate, but was never requested to show it, or any other form of identification.
The airline rules are a bit ambiguous, according to Customer Service at SAS. Infants need to have a passport, but you will most likely not have to show it. But if they ask for it, then you need to be able to show some form of identification, a passport, a birth certificate or something else, if there exist alternatives.
So, according to Customer Service, a birth certificate will suffice, although their rules require a passport.
They did not know what the Norwegian or German authorities rules were, they only recommended to always carry a passport.
The following is a quote from government.no.
Norwegian children must have their own passports. This is also recommended for travel within the Schengen Area (Europe's passport-free zone), since a passport is the only valid proof of identity for Norwegians abroad.
As you can see, the Norwegian authorities recommend that children have a passport within the Schengen Area. They don't say that you must have it.
The following is a badly google-translated text retrieved from konsularinfo.diplo.de:
Children entries are invalid since 26 June 2012 at the parent's passport. Since that day, all children must have (from birth) when traveling abroad its own travel document.
This is for German citizens travelling abroad from Germany, not foreigners entering Germany. However, it's likely that the same rules apply for both.
The following is taken from ec.europe.eu:
Documents for minors
In addition to their own valid passport or ID card, all children travelling:
- alone; or
- with adults who are not their legal guardian; or
- with only one parent
may need an extra (official) document signed by their parents, second parent or legal guardian(s) authorising them to travel.
There are no EU rules on this matter, each EU country decides whether or not it requires such documents.
- The airline company will most likely let you board the plane with an infant without passport.
- The Norwegian authorities recommend, but doesn't require that you bring a passport for an infant
- The German authorities requires that all citizens have passports when travelling from Germany. It's therefore likely that the requirement holds for foreigners travelling to Germany.