I can't answer for Germany, or for selling prints, and my knowledge is only from a Journalist's concern; in the U.S. if it is something/someone that can be seen from the street (public spaces), a release should not be necessary. This may not apply to children. I don't image Germany would be terribly different.
Modeling isn't the same as people in a public space, even if the photos are in public. Since they are presumably doing it for a fee or other compensation (portfolio material), I'd think the release should be part of the contract. Even if they model for free, being a model is a job/career, so I'd think a release would be appropriate.