I think part of it is that some kids, perhaps us as kids, felt more free, we didn't worry too much about what was possible. I think losing the confidence one had as a child isn't that surprising. See, confidence is good, but it can easily become arrogance, and if there was one thing I was as a child, it was arrogant.
The world however, as much as it can end up enabling arrogance, can be brilliant at destroying arrogance.
That and as we get older we have a lot of things to think about, things preying on our mind. Gotta think more about the stories. Take as many stolen moments as we can to dream of these other worlds.
Children can be more free than adults, and I think a lot of the time, we discard the wrong things from our childhood while keeping hold of things like petty jealousy and so on.
Honestly recently things have been a bit rough for me writing-wise, I think in the absence of that grand child-sourced power, there isn't much we can do other than not give up. Just by writing and reading, it is hard not to end up progressing.
Though, in terms of word count, I'm more just struggling against years of actively avoiding anything that even smelt of work. It really is quite impressive how much trouble you can cause for yourself with actions earlier in life. That said, it'll have been two years I've been at this come November.
Take that past me! You never could have done this!
Gotta remember while child me was filled with wonder and inspiration and even dictated a book and a half (then later finished that other book on his own). Teen me just whined about writing.
Oddly I never did come up with a magic system as a kid. Funny that.
Actually though something to take heart in, is that if you did it before, you should be able to do it again.