I've spent a good amount of time thinking about why this change would occur. My current favorite hypothesis is that a playful mode/attitude emerges naturally when you find your circumstances to be 'okay'.
The playful mode seems to be characterized partly by open-ended exploration, and partly by lack of any definition of a 'correct result'.
When we lose the habit of entering the playful mode, it's in favor of another mode which carries out optimization processes on some desired goal state; you lose the 'absence of correct result definition' because there is a definitely some particular result you're looking for; likewise your exploration tends to be highly constrained and goal-directed.
A lot of distillation of Eastern ideas (primarily Buddhist and Taoist ideas) surrounding meditation focus on this distinction between 'modes'. The practice of meditation itself can help to let go of the driven/goal-oriented mind state, but a perhaps bigger part is taking earnest personal account of which things you really need in order to be content. The more things you're able to say you can do without, the fewer reasons you have to stay in the driven, non-playful mind state. (This is not to say you can't have goals: it's just a different way of dealing with them. The recommendation is to not set them up in such a way that things are not 'okay' until they are met.)
Isn't it simply that children have far less preconceptions of the world, as they haven't had time to form them, and that unstructured play is part of how children satisfy the in-built drive for novelty and understanding?
They need to play to make sense of the world. To a child, the most mundane things are genuinely fascinating and novel, as they haven't experienced them much yet.
Psychedelics are one way to reach this state of mind as an adult. They unravel our preconceptions a noticeable amount, transforming the mundane into something novel and fascinating yet again.
I think this perspective, with some effort, can be fostered as an adult as well. Of course, it's a balancing act.
The playful mode seems to be characterized partly by open-ended exploration, and partly by lack of any definition of a 'correct result'.
When we lose the habit of entering the playful mode, it's in favor of another mode which carries out optimization processes on some desired goal state; you lose the 'absence of correct result definition' because there is a definitely some particular result you're looking for; likewise your exploration tends to be highly constrained and goal-directed.
A lot of distillation of Eastern ideas (primarily Buddhist and Taoist ideas) surrounding meditation focus on this distinction between 'modes'. The practice of meditation itself can help to let go of the driven/goal-oriented mind state, but a perhaps bigger part is taking earnest personal account of which things you really need in order to be content. The more things you're able to say you can do without, the fewer reasons you have to stay in the driven, non-playful mind state. (This is not to say you can't have goals: it's just a different way of dealing with them. The recommendation is to not set them up in such a way that things are not 'okay' until they are met.)