A world where people are required to work on improving their skill set for free on their own time is not better than a world where they can receive financial compensation for doing so. Many places allow this and mandate overtime payment for doing so. If someone wants to pay me 1.5x or 2.0x as much for time spent on a task over 40 hours a week that seems like a very appealing prospect to me, especially if it was a task that I was otherwise going to spend my time doing for free.
I think that you're missing the broader point that I'm trying to make here which is this: Why should the state mandate a cap on voluntary employment, rather than focus on ensuring that no one needs to work that much to survive? A system that protects workers from coercion is great. But a system that also prohibits voluntary overcommitment, even when it's for personal growth, artistic mastery, or short-term goals, feels overly paternalistic and your example regarding child labour laws exemplifies that paternalism.
I feel like you're defending the system in Germany not because it's a better system as measured by some objective criteria but because it's the system that you identify with. Is there any sort of data to back up the assertion that a system where people are not allowed to pay other people for more than 48 hours of their time in a week a better system that leads to better outcomes than one where people are free to exchange their time in exchange for a wage with mandatory overtime?