A huge part of this is because you've gotten better! And now, when you're drawing/writing/doing whatever creative task, you're not just mindlessly throwing thoughts at your paper, you're thinking as you do it. Children can churn out a lot more work because it's not yet refined, but when you're older and have more practice, you work with all these thoughts running through your head about form and shape, color palettes or word choice. Now, you're making a dozen decisions with every moment of work, and you're also questioning the decisions you've just made, wondering if you can do it better. Don't beat yourself up about producing less work now than you did back then, because every sentence or shape involves a lot more effort for you now, than it did when you were ten and brand new to this hobby.