First high open space without knee wall

No, with a knee wall it is the opposite: that is, not pulling the hat lower into the face and basically shortening the gable to pull the roof slope into the upper floor, turning it into an attic - but rather, raising the wall plates to bring the attic closer to an upper floor with full standing height. From the room's perspective, a knee wall is somewhat a "dwarf wall by other means." There are regions where one of the two methods is traditionally preferred and the other is so insignificantly uncommon that the terms for the actual "opposites of each other" are used like synonyms. This often leads to confusion when knee wall is said but dwarf wall is meant, or vice versa – I'm used to that already. But your misinterpretation of the knee wall as a term for lowering instead of raising is new to me. Thanks for that – I always like to learn new misunderstandings from lay vocabulary ;-)

I've never heard of drempel and foot purlin before

Show your attic from the inside - then someone can tell you what you have. Pfetten are eaves-parallel "beams" on which the rafters rest; the lower ones are called Fußpfetten (because they are near the foot end of the rafters) and rest directly on the ceiling of the top floor without a knee wall. The consequence of the roof slope then reaching down to the floor is not appreciated in finished attics. Then a wall is placed under another, slightly higher lying purlin (Drempel). Alternatively, the exterior wall of the floor below can be built higher and the Fußpfette then be supported on it (Kniestock). The Drempel is culturally on the retreat. Likewise, in some regions there are traditionally hardly any Kniestöcke; there, the uncommon Kniestöcke are often called Drempel (and then the layman is completely confused).

Our former house had exactly such two children's rooms.. highest point almost 5 meters! I thought it was great

I obviously meant without a knee wall.