I disagree wrt licensed IPs.
There's no single right "amount" of structure to provide to inspire creativity. Different works of art, different moods, different people, different days, will all "want" different starting points. Sometimes you want to experiment with basic geometry and it's useful to start from a bucket of raw parts. Sometimes you want to dig into stories and characters and it's useful to start with something that already exists. And because you can always remove structure from a situation - Legos can always be broken down into the bucket of raw parts! - more highly structured sets strictly expand the available range by pushing out the high end.
Even more than that, kids are learning how to be creative, and having some structure to start from helps a huge amount for a beginner. If you hand a blank piece of paper to someone who's never written anybody and tell them to be creative they'll never even be able to start. If you give them something to start from they'll have a much easier time. Even as expertise builds and you pull back on the guidance it's still incredibly useful to have new material injected from outside your own bubble of experience. Concretely, when you're just getting started it's a hell of a lot easier to learn that you can have a different person win a lightsaber fight than it is to write all your own characters from whole cloth, and when you're trying to learn how people work it's absolutely necessary to examine characters and stories that you didn't make up and that don't just confirm your own preconceptions and biases.
If I wanted to summarize my opinions, I think that playing with licensed-IP Lego sets is actually a critical step in the development of creativity because it lets you get started on more complicated topics earlier, makes you less afraid to adapt existing works, and provides inspiration and outside influence to break filter bubbles. I would compare it in many ways to writing fanfiction, and in a very literal sense it absolutely is.